The singer, who's been opening for T-Swift on her "Redemption" tour, made the announcement on Tuesday morning that she won't be attending the Seattle concert.
Cabello said she began to feel "really sick" after her performance at the Billboard Awards on Sunday and after going to the hospital, "the diagnosis was basically dehydration and I also have a low grade fever."
According to doctor's orders, she needs to rest or she won't get better.
Cabello apologized to fans, writing, "I'm so sorry to let you guys down and I promise I will make it up as soon as I can!"
The "Havana" singer is scheduled to open for Swift alongside Charli XCX through September. It's unclear if she will attend the Denver show on Friday.