The 24-year-old had been in the middle of a Legally Blonde: The Musical performance in Gangnam, Seoul, when onlookers say she appeared "pale and dizzy". Two hours later she collapsed backstage and was rushed to a nearby hospital.
But sources say that after a short overnight stay, the singer is doing "much better".
"It was a scary thing to happen," the source reveals. "The doctor saw to her immediately. She went in to the hospital with her manager, an assistant and [bandmate] Seohyun.
"They said she was dehydrated [and] they put her on an IV drip. Everyone let her rest for a while. By the early hours of the morning she was feeling much better. She had a good sleep."
An SM Entertainment rep had no comment when approached by IdolWow!, however an insider says: "She was run down but feeling fine now. Jessica was given the option to take time off, but she felt fine to return to the apartment, rest some more and then get back to work."
Girls' Generation have a heavy schedule still with their promo for "I Got a Boy". On January 12 the girls will take to the streets of Hongdae to film a "Guerrilla Date" segment for KBS2's Entertainment Relay.