After Gomez appeared on "The Tonight Show" Thursday, her representative said the Disney star complained of headaches and nausea and was taken to an area hospital.
The 18-year-old singer and actress remains at the hospital, undergoing routine tests. The source of her sickness was not known at the time of this report.
Her Friday appearance was to promote her new movie, "Monte Carlo." Many of her fans gathered at Santa Monica Place, some traveled a long way to see her.
"We came from San Jose," said 12-year-old Linda Yao. "And it took seven hours to see Selena, but she's not here. It's pretty sad."
"We got pulled out of school for this," said 13-year-old Ashley Napier. "We heard about this at least two weeks ago and we were really excited. I heard she wasn't here and we almost started crying. It was really sad."
Even though Gomez was unable to attend, a lot of the activities were still available for fans, such as getting their hair and makeup done and posing for photographs.
Although fans were disappointed they said that they hope she is feeling better. The producers of "Monte Carlo" say that they hope to schedule another event when she is feeling better.