Kristin Cavallari feels ''too thin''.

The former 'The Hills' star has revealed she is trying to bulk up after giving birth to her sons, Camden, two, and eight-month-old Jaxon, with her husband Jay Cutler, as she doesn't like looking too skinny.

She said: ''I don't know why or how but after I had children my body literally changed and now, for me, it's about trying to keep muscle on so I don't look too thin.

''Because it's very easy, especially because I have no curves, that I can look really skinny and I don't like that look.''

The blonde beauty, 28, insists there is ''no secret'' to her impressive figure and tries to ''workout four to five times a week'' and stick to an ''incredibly healthy'' diet in order to stay in shape.

She told E! News: ''There's no real secret, I work my butt off in the gym...[but] I'm trying to gain some muscle so I've been working out with really heavy weights...and then I drink a protein shake right afterwards.''

Kristin is also helped by her NFL quarterback husband, 31, who is always giving her ''workout tips.''

She said: ''He helps with giving me certain moves. Like I want to make my butt bigger so he's telling me what to do.''