"Its my birthday and ill get crunk if I want to! #ibiza," she tweeted on Thursday
Taking to Twitter, she told fans: "So sorry I had to cancel at Guilfest guys. I was really unwell. Hate letting down my fans but I genuinely couldn't perform. I'm on the mend X"
The X Factor judge was joined at the Liverpool boot camp by fellow judges Nicole Scherzinger, Louis Walsh and Gary Barlow.
>IN PICS: 24 HOURS IN CELEBVILLE
Celeb Pics Of The Day
(01 of11)
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Rihanna tweets a sexy snap of her and pals on holiday (credit:Rihanna/Twitter)
(02 of11)
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Paloma Faith at the VIP launch of Audi Digital Store at Audi City (credit:WENN)
(03 of11)
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Caroline Flack arrives at Liverpool Lime Street ahead of the first day of The X Factor bootcamp. The bootcamp has been moved to Liverpool for the duration of the Olympics being held in London. (credit:WENN)
(04 of11)
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Chris Eubank out and about in Mayfair London (credit:WENN)
(05 of11)
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Miley Cyrus exits her Winsor Pilates Class in Los Angeles, California (credit:WENN)
(06 of11)
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Marion Cotillard at The Dark Knight Rises New York Premiere (credit:WENN)
(07 of11)
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Janice Dickinson walks through a sunny West Hollywood with a friend Los Angeles, California (credit:WENN)
(08 of11)
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Alexandra Burke at Disney's 'Let It Shine' TV Premiere at The May Fair hotel London, England (credit:WENN)
(09 of11)
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Harry Derbridge Disney's Let It Shine TV Premiere at The May Fair hotel (credit:WENN)
(10 of11)
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Holly Willoughby channels Mad Men in sexy new shoot (credit:PA)
(11 of11)
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Rihanna's on her hols in Sardinia (credit:Rihanna/Twitter)