Selena Gomez Selena Gomez

Selena Gomez says exhaustion and malnourishment was to blame for her hospitalization.

"I was just very malnourished, so I was low on iron and exhausted," she told reporters after her rescheduled show Monday.

The 18-year-old Gomez, who was hospitalized Thursday night,

tweeted earlier Monday that she was ready to perform: "Feeling much better, thank you all for being so understanding. Can't wait to see you all today at Santa Monica Place for the Monte Carlo kickoff." 

Selena Gomez hospitalized

She later took to the Santa Monica stage and told more than 500 fans: "I want to thank you so much for coming ... If you guys came on Friday, I'm so sorry that I couldn't be here. ... I love you guys very much."

Singing her hit song "Who Says" and "A Year Without Rain," Gomez told the crowd, "I do feel better!"

The concert is part of a promotion for Gomez's upcoming film Monte Carlo, out July 1.