The St. Louis Post-Dispatch reports Cabello left the Hollywood Casino Amphitheatre show shortly after performing "Bo$$" with several songs left in the set. One of her bandmates blamed a wardrobe malfunction at the moment, though Cabello never returned.
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Cabello later tweeted she just wanted to sleep for three days.
Cabello previously told Billboard she started having anxiety problems last year.
"I was having terrible anxiety, nonstop," she said. "My heart would beat really fast the whole day. Two hours after I woke up, I'd need a nap because my body was so hyperactive. It was so eff -- sorry, but it was so f--ed up. I was scared of what would happen to me, of the things my brain might tell me. I realized the stuff I thought was important isn't worth my health. Now I write in a diary every day, work out and meditate."
Watch Fifth Harmony finishing the concert without Cabello below.