October 03, 2012 01:00 PM

Janet Mayer/Splash News Online

At last night’s ULTA Donate With a Kiss event in N.Y.C., we couldn’t help but notice new mom Kristin Cavallari‘s svelte new shape.

Just eight weeks after welcoming son Camden Jack, the 25-year-old debuted her slimmer figure in a black and leather Rubin Singer sheath paired with slinky heels while helping promote the company’s breast cancer charity.

So how is the star losing the baby weight?

“I honestly can say I owe it all to breastfeeding,” she tells PEOPLE Tuesday.

“I read somewhere that it’s like running six miles a day. I know nursing is difficult for a lot of moms so it’s easier said than done. I eat healthy in general and I’m taking it easy in the gym and working out three to four days a week.”

But according to Cavallari that doesn’t mean she’s beach-ready — yet.

“I do want to tone up my stomach a little bit more. Trust me, if you saw me I’m not ready for a bikini, let’s just put it that way,” she says with a laugh. “I’m close, but I haven’t [lost all the weight].”

— Shanelle Rein-Olowokere with reporting by Lilly Workneh

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