Joe Jonas checked into an emergency room in L.A. while doctors inspected his abs -- for severe stomach pains -- 48 hours after Joe did some late night partying in Chicago.
A rep for Joe confirms he went to the emergency room at Children's Hospital L.A. on Sunday. We're told Joe was only at the hospital briefly ... and it's not clear what was wrong with him.
One potential hint -- on Friday night (into Saturday morning) Joe was out at a Chicago hot spot Board Room Nightclub (above) partying ... and performed the next night in Chi-Town.
Whatever the problem was ... Joe felt good enough to eat out -- we spotted him and his bro Nick leaving Katsuya last night. Sushi after stomach troubles? Risky, Joe.
Joe Jonas, 21, Kevin Jonas, 22, and Nick Jonas, 18, all flexed their evangelical Christian dandy boy band muscles at the same event in Costa Rica on Tuesday.
Joe Jonas is prepared to go to war over a blog posting that insists he and his underage ex-girlfriend Demi Lovato are currently "friends with benefits" ... in fact, Joe's lawyers are already on the attack.
TMZ has learned ... Joe's attorney just fired off a letter to a Canadian entertainment blogger named Zack Taylor ... blasting the guy for a story he posted which implied that 20-year-old Joe and 17-year-old Demi "are involved in an illicit relationship."
In the letter, obtained by TMZ, Jonas' attorney claims the story is "completely false, offensive and defamatory" -- and accuses Taylor of a "malicious attempt to attack and devalue [Joe's] good name and reputation."
Team Jonas is demanding that Taylor pull the post ASAP ... or else a lawsuit may follow.
Taylor tells TMZ, "We are currently reviewing the situation & letter. However, at this time we do stand by our sources and their claims."
Before Justin Bieber went toe-to-toe with a photographer -- sparking a police investigation -- he trained with "Iron" Mike Tyson! Wondering how they ended up together? Well, we got Tyson on the phone ... and you won't believe his excuse for kicking it with Biebs. Plus,…