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It's no secret that Kristin Cavallari looked insane just two months after giving birth to her second son, Jaxon Wyatt, who is now 8 months old. 

So, when E! News caught up with the 28-year-old mother of two at the lookbook shoot for her latest shoe collection with Chinese Laundry, we had to ask the former Hills star how she stays in shape while balancing her busy schedule—and we love that Cavallari isn't afraid to admit she works hard for her bangin' bod. 

"There's no real secret, I work my butt off in the gym," the fashionista revealed to E! News. "I woke up at 5:30 a.m. and went and walked uphill on the treadmill for 30 minutes." 

Talk about dedication! As for the blond beauty's everyday diet and fitness regime? 

WATCH: Kristin Cavallari is writing a book!

"I try to work out four to five times a week and I eat incredibly healthy," she shared. "I'm the kind of person that does not eat any toxic chemicals…I just try to eat real food. I think probably the best tip is that I try to always get a vegetable in somehow, even in the morning if you're making a smoothie, you can throw some spinach in it and it's an easy way to get a serving of greens." 

In addition to dishing on her diet, Cavallari, who is no stranger to workout selfies, admitted she's currently "trying to put on muscle" or order to bulk up her super-skinny frame. 

Kristin Cavallari

Instagram

"I don't why or how, but after I had children, my body literally changed. And now for me, it's about trying to keep muscle on and gain muscle, so that, yeah, I don't look too thin," she explained. "It's very easy, because I have no curves, that I can look really skinny and I don't like that look. So, yes, I'm drinking protein shakes, I'm trying to put on muscle and I'm lifting very heavy weight right now." (Press play to find out if she's trying to achieve a booty like Kim Kardashian!)

VIDEO: Kristin Cavallari talks Laguna Beach love triangle

Kristin Cavallari

Valerie Macon/Getty Images

But it wasn't all diet and fitness during K.Cav's chat with E! News—the designer also shared whether she keeps in contact with her former Hills co-stars. 

After revealing she talks to Heidi Montag "all the time," Cavallari was asked about her relationship with former on-screen enemy Lauren Conrad

"We've never kept in touch. It's not a shock to anyone, I'm sure," she said of her erstwhile Laguna Beach co-star before joking, "Breaking news!" 

"I wish her well, of course," she added. "And if I saw her, I would give her a big hug." 

But what about Spencer Pratt, Audrina PatridgeWhitney Port or Stephanie Pratt? Watch the clip to hear what she has to say! 

PHOTOS: Kristin Cavallari's mommy style