A representative for the 21-year-old says in a statement that Cyrus is canceling her Wednesday performance at the Scottrade Center in St. Louis.
The singer also tweets that the hospital will not release her. She apologized in a tweet Wednesday.
Cyrus canceled a Tuesday show in Kansas City, Mo. Last week, she canceled a stop on her "Bangerz" tour in Charlotte, N.C., because she had the flu.
Her rep said Wednesday that Friday's show in Nashville, Tenn., "is still scheduled to go on as planned."
Tickets for the St. Louis concert are being refunded.