The 'We Found Love' singer was forced to cancel a concert in Malmö on Monday night after seeking medical treatment for her illness.
Rihanna tweeted a photo of herself hooked up to an IV drip in the hospital along with an apology, saying: "Sorry, Malmo!!!"
In a longer statement, she explained: "I am sorry to everyone who was coming out to my show in Malmo... I was so excited to perform for you all."
"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."
Meanwhile, a spokesman for concert promotions company LiveNation told TMZ that Rihanna is "in no condition to conduct the concert".
Rihanna has been confirmed to appear at the 2011 Jingle Ball to be held at London's O2 Arena alongside Pixie Lott, Olly Murs and The Wanted.
She has also collaborated with Drake on the title track from his forthcoming sophomore LP Take Care, due in stores on November 15.
Watch Rihanna's video for 'We Found Love' below: