The 18-year-old had complained of nausea and a severe headache whilst filming The Tonight Show with Jay Leno on Thursday, and was admitted to hospital later that night.
But taking to the stage at Santa Monica Place outdoor mall on Monday - an event which had been rescheduled from Friday - the singer and actress told People magazine, 'I was just very malnourished, so I was low on iron and exhausted.'
Addressing her 500 fans, she added, '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.'
She then launched into a performance of Who Says and A Year Without Rain and told the crowd, 'I do feel better!'