The 21-year-old singer announced the news in a tweet Monday after falling ill following the 2018 Billboard Music Awards. Cabello was to open for Swift Tuesday, May 22 in Seattle, Wash.
"so yesterday after my performance at the billboards, i was feeling really sick and ended up in the hospital to get checked out. they ran a lot of tests and everything is fine, but the diagnosis was basically dehydration and I also have a low grade fever," Cabello wrote.
"the doctors have told me I really have to get rest otherwise i won't get better - so unfortunately I won't be able to perform my set during the Reputation tour in Seattle," she said. "I'm so sorry to let you guys down and I promise I will make it up as soon as I can!"
Cabello performed "Sangria Wine" and "Havana" at the Billboard Music Awards on Sunday. She thanked fans for caring about her health while accepting the Billboard Chart Achievement Award at the ceremony.
"I really just want to thank my fans for sticking by me and my Camilizers for sticking by me and loving me and caring for me and asking if I get enough sleep and drink enough water," the star said. "I love you guys so much."
Cabello is scheduled to perform at Swift's show Friday, May 25 in Denver, Colo.