Rihanna -- K.O.'d by Flu Virus in Sweden

Rihanna is currently hooked up to an IV drip in Sweden ... and has canceled a performance scheduled for tonight ... blaming the whole thing on a case of the flu.

Rihanna was scheduled to perform in Malmö, Sweden -- but a rep for Live Nation claims the show has been canceled because the singer is "not in condition to conduct the concert."

Rihanna tweeted an apology an hour ago, writing, "Sorry Malmo!" -- attaching a picture of her arm with tubes coming out of it.

The singer also released a statement saying, "I am sorry to everyone who was coming out to my show in Malmo [Sweden]. I was so excited to perform for you all."

She adds, "It would have been a great time…so much better than being sick with the flu, ugh! I'm really disappointed I couldn't be there."