09/04/2012 AT 07:30 PM EDT
knows exactly why she gained the weight
she did while pregnant with her daughter Maxwell Drew
"I let myself indulge in everything I wanted because it was the first time I was ever pregnant, and I wanted to enjoy it," Simpson, 32, tells USA Today
That included eating one of her faves, macaroni and cheese. But Simpson didn't quite understand how the indulgence would affect her post-baby body. "I didn't realize [the weight] didn't all come off with the baby," says Simpson.
She's now given up many of those foods, however, while doing Weight Watchers as the company's newest spokeswoman – she appears in the ads next week.
She hasn't revealed her current weight, nor what she weighed at her heaviest, but says she's lost weight each week on the program. Simpson says she’s allotted 28 points a day since weaning Maxwell, down from 34 points while nursing.
She also works out with personal trainer (and PEOPLE.com blogger
) Harley Pasternak four or five times a week, mostly walking
now, says Simpson, because, "my boobs are way too big to run at this point."
Numbers aside, the star seems realistic. "My body is not bouncing back like a supermodel," says Simpson. "I'm just your everyday woman who is trying to feel good and be healthy for her daughter, her fiancé and herself."
Simpson, who is engaged to former NFL player Eric Johnson, will reveal her post-baby body
on Katie Couric's new show, Katie
, on Sept. 10.