Kim Kardashian has been busting her booty at the gym and watching what she eats, and four months after giving birth to her first child, she wants to show off the results! We have to say, we're pretty impressed, especially when you consider she gained a reported 60 pounds during her pregnancy!

The curvy reality starlet posted this sexy swimsuit selfie on Instagram last night and simply wrote "#NoFilter," and when asked what her slim down secret was on Sunday via Twitter, Kim replied, "I've actually been doing the Atkins diet & luv it."

"She is eating lots of lean proteins, healthy fats like nuts and avocado, carbs, fruits, veggies and cheese," a rep for Atkins told Us Weekly. "She wanted to be a healthy, nursing mom and not toxify her milk with her diet or lose weight too quickly. She's been doing Atkins since her doctor approved it at the end of June."

We're guessing Kanye loves it too, because he re-tweeted her pic and wrote, "HEADING HOME NOW. Ha!