Joel Madden and Nicole Richie switch on the lights at UNICEF’s Light The Night Christmas lighting ceremony in Beverly Hills, California - (11/22/08).

Nic is looking really thin again....

BAC/ WENN

Nicole Richie tries to hide from the photographers as she leaves a restaurant in Malibu with boyfriend Joel Madden in Los Angeles, California - (09/01/08).

Touring? Mary-Kate Olsen? Him just being Joel Madden? (LEAP)

Is Nicole being shortchanged?

A source told Star: "When Harlow was born, Joel said he wouldn't tour so much, but he hasn't slowed down at all. It's been hard for her to balance a career and take care of a baby by herself."

Nicole Richie doesn't ever want to be fat again and that's why she eats sushi and salad all day. She also hasn't let having a baby stop her from going out to the bars and clubs.

Apparently this isn't the only thing keeping Nicole tiny.

OK! can reveal the star's slimming secret isn't some miracle diet or fitness routine, but waist-cinching undergarment Spanx!

"She sometimes wears two pairs at a time," a pal of Nicole reveals to OK!.

And while the new mom is keeping this secret from the public, her fiancé Joel Madden, 28, is all too familiar with them.

"He'll help her find them in her closet if they're running late," says the friend.

"But he thinks she looks great just the way she is."

Nicole Richie, 26 and her boyfriend Joel Madden, 28 welcomed a baby girl on Friday.

Harlow Winter Kate Madden was born at Cedars-Sinai Medical Center and weighed 6 lbs. 7 oz.

The couple's rep said: "The beautiful healthy baby girl left the hospital with her ecstatic parents."

Currently the new parents have yet to set a wedding date but have decided to put their baby first.