12/21/2007 AT 08:35 AM EST
Things are even more hectic than usual for Britney Spears
. On Thursday, after spending a cozy afternoon at her Beverly Hills home with sons Preston, 2, and Jayden, 1, the singer scheduled – then canceled – a session at Millennium Dance Complex.
"She wanted to try out a new dance routine," says a source. But, in the wake of her 16-year-old sister's surprise pregnancy announcement
, the studio was swarmed by hoards of photographers.
When Spears, 26 – who had been kept in the dark about the baby news until the story broke on Tuesday – saw the crowd, "she freaked out and just kept driving," says the source, who was on the scene.
"Britney said that she doesn't want to deal with the media because she is still shocked [about Jamie Lynn's pregnancy news]," adds the source. "She feels that she needs peace and quiet."
Spears, however, was friendly with the photogs during a shopping trip after the scene. Although the area offers plenty of great shops, Spears stopped at a random souvenir store on Hollywood Boulevard, where she picked up (and posed with) a Tinkerbell T-shirt and a pink bedazzled onesie.
On the way home, Spears also hit Carl's JR for a late night snack: cheeseburger with a large fries and coke.