Kristin Cavallari "Has Already Lost All of Her Baby Weight"

Kristin Cavallari takes newborn son Camden Jack Cutler for a stroll in Chicago, IL on August 8th, 2012. Credit: Michael Simon/

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. 8.

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."

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

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