Kristin Cavallari wasted no time losing the baby weight!

The 25-year-old former Hills star welcomed her first child, son Camden, with fiance, Chicago Bears quarterback Jay Cutler, 29, on Aug. 10.

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Just five weeks after giving birth, Cavallari "has already lost all the weight by working out like crazy with a trainer," a source tells Us Weekly.

Last week, the reality star tweeted, "Walking uphill on the treadmill to work out this booty. Bye bye baby weight."

PHOTOS: Kristin's best bikini moments

While Cavallari has had no problems losing the weight, there is one thing she's struggling to change after becoming a mom.

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"Still need to work on my potty mouth," she tweeted on Sept. 15. "Swearing just comes so naturally to me."

This article originally appeared on Usmagazine.com: Kristin Cavallari "Has Already Lost All of Her Baby Weight"