There's a reason Rihanna's arm was hooked up to a bunch of needles and tubes yesterday -- sources close to Rihanna tell TMZ, the singer had been struck down with a bad case of the flu ... and required an IV drip.

Sources tell us, she started feeling really ill last week and saw a doctor on Thursday ... but by the time of her "Saturday Night Live" appearance, she still wasn't fully recovered.

As we first reported, Rihanna missed the SNL dress rehearsal because she was sick -- but show producers didn't buy her excuse ... insisting she didn't appear ill.

But looks might be deceiving -- because we're told Rihanna wasn't just ill during her SNL performance, she also continued to feel sick the next few days ... and her doc was forced to put her on Penicillin.

Soon after, Rihanna began to experience flu-like symptoms -- requiring IV treatment yesterday, which is when she posted the above picture on Twitter.

We're told she's feeling much better now.

Rihanna was a no-show for her final rehearsal for "Saturday Night Live" last night -- which did not sit well with the show's producers, including head man Lorne Michaels ... TMZ has learned.

According to our sources, RiRi called in sick for the dress rehearsal, which is done live in front of a studio audience right before the actual show. We're told Michaels takes dress rehearsals very seriously and was quite upset when Rihanna bailed at the last minute.

Our sources say that in her place, Rihanna's choreographer danced to her songs. 

When Rihanna showed up later looking just fine, we're told the staff was shocked ... and felt she didn't appear sick enough to have missed the dress rehearsal ... though no one from the show called her on it.

According to our sources, once the live show got going, Rihanna was seen eating an apple and walking around in between songs ... which bothered several staffers because she did not seem ill at all.

We're told producers can't even remember the last time someone has skipped the dress rehearsal.

Sources close to Rihanna tell TMZ she went through every rehearsal except the dress rehearsal (which she was in the building for) because she just felt too ill to do it. We're told RiRi had a doctor come to her dressing room before the show and administer a shot of penicillin before she went on. 

A rep for the singer tells TMZ, "Rihanna was sick and needed to save her voice for the live show."

Lindsay Lohan was all set to continue her obsession with Marilyn Monroe during last week's "Saturday Night Live" -- but a sketch featuring her and Jon Hamm was scrapped at the last minute ... TMZ has learned. 

According to our sources, the sketch was to feature Lindsay as Marilyn and Hamm as JFK appearing to Rick Santorum (played by Andy Samberg) while he is at a conservative event (see photo above).

During the sketch, Marilyn and JFK explain Marilyn was on birth control provided to her by Medicare, which causes Santorum to vomit uncontrollably. Santorum (the real one) recently said that JFK's speech from 1960 about keeping religion out of politics "makes [him] want to throw up."

In the end, we're told the sketch made it all the way to rehearsals, and was even performed in front of a live audience, but was ultimately cut because of time constraints.

Lindsay has channeled Marilyn several times over the years, including her most recent Playboy cover.

Love it or hate it, there's at least one person who thinks Lindsay Lohan did a great job on "Saturday Night Live" last night ... and that's Lindsay herself.

Sources close to Lindsay tell TMZ she understands it wasn't a perfect show, but she feels proud of the episode. We're told there were several last minute changes -- including ditching her singing monologue in favor of one with cameos from Jimmy Fallon and Jon Hamm (see below).

Our sources say Lindsay was excited to be back working and really missed being a part of the action. 

According to our sources, Lindsay understands people are going to bash her performance regardless ... but she feels she did the best she could do.

So we gotta ask ...

Lindsay Lohan was seen leaving her New York hotel this morning, script in hand, as she gets ready to host "Saturday Night Live" tonight.

As TMZ previously reported, LiLo plans to sing her opening monologue and from there ... nothing is off limits. We're also told Lindsay has invited several famous friends to cameo on the show, but no word on exactly who will make an appearance. 

Lindsay is counting on tonight's "SNL" to re-launch her career. So we gotta ask ...

Lindsay Lohan's taking her music career to the next level on SNL tonight -- TMZ has learned, the actress plans to SING the majority of her opening monologue on the sketch show.

Sources tell TMZ, nothing's off the table either -- Lindsay will poke fun at all the recent mishaps in her life ... arrests, endless court dates, necklace-stealing sagas, jail, morgue duty ... the list goes on.

But Lindsay won't be alone on stage -- we're told several cast members will join her in chorus.

Sources tell us, Lindsay wants to show the world she doesn't take herself as seriously as people think -- and actually has a sense of humor about the legal troubles that have plagued her the last few years.

We're told Lindsay has also extended several personal invites to famous friends to join her on the show -- and they unanimously accepted. No word yet on who the surprise guests might be.

Bottom line ... Lindsay knows there's a lot riding on tonight's performance -- so she's determined to make it one for the ages, practicing non-stop all week long.

Speaking of singing -- let's all take a moment to remember Lindsay's foray into pop music ...

Lindsay Lohan has been the butt of jokes on "Saturday Night Live" for years, but TMZ has learned that this coming Saturday ... the LiLo jokes will be made by Lindsay herself. 

As TMZ first reported, Lindsay has been dying to do "SNL" as a way to re-establish her career and reached out to Lorne Michaels personally to make it happen.

According to our sources, Lindsay plans on using all the unpleasantness from the last few years as fodder for the show. We're told Lindsay has already told "SNL" that she is game for anything and nothing is off limits.

Our sources say Lindsay wants to show that she is "in on the joke" -- so expect to see references to any number of DUIs, alleged necklace thievery, jail stints, mug shots ... and on and on. 

Lindsay Lohan was DYING to do "Saturday Night Live" again -- and kickstart her comeback -- so she reached out to Lorne Michaels herself to book the hosting gig ... TMZ has learned.

Sources close to Lindsay tell TMZ she has always felt the "SNL" executive producer has been very fatherly towards her throughout the years. We're told LiLo has always enjoyed doing the show, but was waiting for the right time to do it again. 

So now that her community service is coming to an end, our sources say Lindsay called up Lorne and asked him to consider her for the show again. We're told they had multiple conversations and eventually settled on a date ... March 3.

According to our sources, Lindsay feels doing "SNL" again will remind people she's an actress ... not a defendant.

Daniel Radcliffe is coming to the defense of the singer he introduced on "Saturday Night Live" this weekend ... sorta.

Radcliffe was at the Bafta nominations ... when he was asked about Lana Del Rey -- who's been TRASHED by music critics and people with ears alike who suffered through her baffling performance on the show (below).

Radcliffe responded, "It was unfortunate that people seemed to turn on her so quickly."

He added, "I also think people are making it about things other than the performance. If you read what people are saying about her online, it's all about her past and her family and stuff that's nobody else's business."

Radcliffe concluded, "I don't think it warranted anywhere near that reaction."

Alec Baldwin took even more unrelenting shots at American Airlines last night -- mocking his recent cell phone debacle on "Saturday Night Live" ... even referring to himself as an "American treasure."

Among the awesomeness ... Alec -- pretending to be the pilot of his ill-fated flight -- explained why he slammed the lavatory door so loudly during his alleged standoff with a flight attendant ... claiming, "Words with Friends" can be "frustrating."

Watch the clip.

"Saturday Night Live" legend Chevy Chase believes four SNL cast members truly stand out as the best ever -- and one might shock you ... because rumor has it ... Chevy despises the man.

Here's another hint -- two are dead.