Selena Gomez performed a concert at Santa Monica Place Monday for hundreds of fans--just three days after a mysterious hospitalization. Monday's concert was rescheduled from Friday when she'd canceled due to her hospital visit.

"I want to thank you so much for coming," Gomez told her fans at the the concert, PEOPLE reports. "If you guys came on Friday, I'm so sorry that I couldn't be here. ... I love you guys very much."

When asked by reporters why she'd been in the hospital, the 18-year-old singer said, "I was just very malnourished, so I was low on iron and exhausted." After performing several songs, she also told the audience, "I do feel better!"

The actress and singer was rushed to the emergency room after a visit to 'The Tonight Show' last Thursday, after complaining of nausea and a severe headache, according to TMZ. Gomez was reportedly feeling ill during the taping but stuck it out like a pro.