"Britney was supposed to wear a matching corset that she opted out of wearing at the last minute," a source tells PEOPLE. "She was just plain nervous because of all the hype and she's embarrassed."

Adds another, "She looked disappointed" backstage after the performance.

"She was extremely nervous. It's been awhile since she's done this, and she was performing in front of all her peers, on live TV. She just got out there and froze," says a source who was also backstage as well as at rehearsal. "People who were at rehearsal saw something else. It was good. It really was."