Gomez was reportedly "feeling horrible during and after the show, complaining of nausea and a severe headache."
She is still at the hospital today, where she is undergoing "routine" tests. The exact nature of her illness has not been disclosed.
A planned Gomez appearance to promote her film "Monte Carlo" in San Monica today has been cancelled.
While boyfriend Justin Bieber has not commented on the news, "#prayforselena" and "#getwellselena" have already shot up to Twitter's trending topics list.