Britney Spears may have been all smiles as she took centre stage for the annual Christmas tree lighting ceremony at NYC's Rockefeller Center but behind-the-scenes things were less rosy, according to insiders.

After an underwhelming performance on Good Morning America, the singer had been set to perform her switching-on duties live but backed out at the last minute.

"She was originally supposed to perform live and then she wanted to pre-tape her performance instead," the source tells OK!.

"Then she didn't want to do that. She keeps changing her mind and now she's not performing at all."

After multiple takes, Britney eventually completed her "Happy Holidays" message and hightailed it to the Rainbow Room for a slap-up birthday meal with friends, family and entourage.

At one stage in the evening Britney is said to have tried her best to comfort youngest son Jayden who had begun crying.

"Jayden kept screaming 'Daddy', it was so sad," says an eyewitness. "She asked for a high chair. Jayden was throwing a tantrum so Britney picked him up and walked outside.

"As she did, she literally snapped her fingers at her security guards who immediately got up and followed her outside."

Things apparently took a turn for the better when Brit and her group hit Club Tenjune for an alcohol-free party.

And, according to onlookers, the singer took the opportunuty to let her hair down after a day of professional engagements.

"She was laughing and grinding with her friends," a fellow partygoer tells OK!.

"The bodyguards formed a circle around her and she danced with her assistant and dancers. She was tossing her hair that had fallen out of a bun and really getting into the music and having a good time."

See Britney's birthday in pictures below