Christina Aguilera's Slim-Down Secret Weapon Revealed

03/29/2013 at 09:45 AM EDT

Christina Aguilera with Tee Sorge inset

Jason LaVeris/FilmMagic; Inset: Chris Tomko/Bravo

Though simple hair and pretty makeup helped soften her look, Christina Aguilera is definitely showing off a harder body these days.

When it comes to how many pounds she's lost, The Voice star who once declared, "I don't weigh myself" hasn't said. But a source close to the singer confirms she is working out at home regularly and "exploring new methods of exercise" with different trainers, including weight training with fitness guru Tee Sorge.

She's also "getting into healthier, fresh eating," and that includes trading in her usual burger for seared tuna salad or sushi at her favorite L.A. restaurant, Hillstone.

But no matter what her dress size, "Christina is generally very comfortable with her body," says a second source, who recently observed her at a photo shoot. "She seemed to have a strong sense of what she could pull off that day."

As Aguilera herself has said, "I happen to be very confident in my own skin."