Camila Cabello has pulled out of performing at Taylor Swift's Reputation tour in Seattle due to dehydration.
Camila says she was checked in hospital after performing at the Billboard Awards - and has been told she also has "a low grade fever".
The former Fifth Harmony star tweeted: "I guess sometimes I just push myself too hard".
Camila is due to perform at the BBC's Biggest Weekend in Swansea this coming Sunday, 27 May.
Newsbeat has contacted Camila's manager to see if her appearance is still going ahead.
Camila and Charli XCX are the support acts for Taylor's 51-date tour - which started on 8 May in Arizona.
Camila continued in her statement: "I'm so sorry to let you guys down and I promise I will make it up as soon as I can".
She picked up the Billboard chart achievement award on Sunday night in Las Vegas.
Camila says doctors have told her "to get rest otherwise I won't get better".
Speaking to the Radio 1 Breakfast Show with Nick Grimshaw in March, when her Biggest Weekend appearance was revealed, she said: "I'm so excited. It's going to be my first time going there [Wales].
"Friends of mine have played at this before and it just takes over the internet and I've just watched from afar.
"I'm excited to be doing that and I'm excited to be going to Wales because I've never been before."