Bristol Palin is admitting she had some work done to her face -- but insists it wasn't cosmetic.

Sarah Palin's 20-year-old daughter had everyone screaming "Plastic surgery!" after she showed up to the White House Correspondents Dinner last week with a slimmer physique and face -- but she tells Us "It's not plastic surgery. I had corrective jaw surgery."

She also tells the mag "It improved the way I look, but this surgery was necessary for medical my jaw and teeth could properly realign...I don't obsess over my face.

Bristol likes the new look though -- saying "I look older, more mature, and don't have as much of a chubby little baby face!"

Check out Bristol's changing face above!