Credit: Gries/MTV/PICTUREGROUP; courtesy of Portwood

Whoa, mama!

Teen Mom star Amber Portwood says in the new Us Weekly (out now) she's "been chunky my whole life." That's a thing of the past: since giving birth to daughter Leah (with boyfriend Gary Shirley, 23) in November 2008, the MTV reality star has lost an impressive 65 pounds!

"I was looking in the mirror and I thought, I don't like this. I'm only 20. I need to kick it into gear," the 5-foot-4 Indiana native, now 118 pounds, tells Us Weekly of her post-baby body.

How did she do it? Out with mac and cheese and sweets and in with lean meat, fruit and vegetables -- plus two-hour cardio and weight sessions four times a week. (The Krav Maga disciple is even gearing up for her first martial arts competition.)

"The first time I tried on jeans after I lost weight, there were a size one," Portwood reveals. "I thought, there's no way. This is crazy!"

"I'm more comfortable in my skin," she adds, noting that her beau Gary "really likes" her new shape. "Who wouldn't?"