Pretty hard to believe Kristin Cavallari was ever pregnant!

The 25-year-old star of "Laguna Beach" and "The Hills" debuted her post-baby bod on the red carpet last night, stepping out to her first public event since giving birth to baby Camden two months ago.

Cavallari hosted ULTA Beauty's 'Donate With a Kiss' event at the James Hotel in New York on Tuesday night, looking skinnier than ever in a super-tight black cocktail dress.

So ... what's her secret?

"I started working out when he was four weeks old," she told reporters last night. "I'm just working out by myself and doing 20 minutes of cardio, 30-40 minutes of light weights. Nothing crazy, just three or four times a week."

"I remained healthy during my pregnancy, I didn't use it as an excuse to just completely go crazy so I think that's why it's been kind of easy for me to lose most of the weight," she adds.

That statement is in far contrast to fellow new-mom Jessica Simpson, who admits she went overboard on macaroni and cheese.

"I let myself indulge in everything I wanted because it was the first time I was ever pregnant, and I wanted to enjoy it," she told USA Today. "I wanted to be happy and eat what I wanted."

Check out more shots of Kristin's amazing physique in the gallery above!

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