Nicole Richie: Post-Pregnancy Blues?

New mom Nicole Richie stepped out for the first time on Jan. 25 since giving birth to her baby daughter Harlow. At her side was her baby's daddy, Joel Madden, who dutifully held her hand as they made their way out of a private party in West Hollywood for Nicole's manager, Benny Medina.

But sources at the party say Nicole — who usually loves the spotlight — was uncharacteristically somber that evening, and now friends worry she may be having a hard time adjusting to being a new mom.

"Nicole's situation is reminicent of that episode of Sex and the City where Miranda wants to be a good mom but isn't sure how to do it exactly," Nicole's friend tells OK!.

The 26-year-old Simple Life star is also said to be stressing over losing her baby weight. The notoriously pin-thin Richie looked amazingly svelte at the West Hollywood party in a body hugging black dress. (She gave birth just two weeks earlier!)

"She keeps a food diary," a source reveals to OK!. "She writes down everything she eats and figures out where she can cut calories." Confirms another source, "She tries to joke about counting her caloric intake, but everyone knows she's very serious about getting her body back."

For the full story on how Nicole is coping with motherhood, pick up the new issue of OK! — on stands everywhere Thursday!