Kristin Cavallari Explains Her Weight Issues

— -- It's hard to believe that Kristin Cavallari has given birth to two children.

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

The former "Hills" star, 28, said she works out four to five times a week and eats very clean.

"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," she said.

But with all the hard work and weight loss comes a downside. Cavallari said she actually wants to pack on some muscle.

"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 added. "Because I have no curves ... 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."

Last year, Cavallari told E! that she dropped 20 pounds only two months after giving birth to her second son, Jax, in May. Cavallari and her husband Jay Cutler are also parents to son Camden, 2.