The Disney star reportedly became severely nauseous after her interview with Jay Leno on The Tonight Show and was taken to hospital on Thursday for tests.
The cause of her illness remains a mystery.
Earlier, the 18-year-old singer-actress had been in high spirits as she discussed her relationship with teen pop star Justin Bieber.
Gomez revealed that she had introduced Bieber to her mother, Mandy, who had given the 17-year-old Never Say Never star the thumbs up.
'He passed the test,' a smiling Gomez told Leno.