The "Disturbia" singer was forced to cancel her show in Malmo in southern Sweden.
"I am sorry to everyone who was coming out to my show in Malmo," she said in a statement, tweeting to fans a photo of her with an IV inserted in her arm.
"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," the 23-year-old added.
There's no word yet on whether the Grammy winner will be able to make her performance in Stockholm Tuesday night.