No, that’s not a flotation device. And yes, Khloé Kardashian’s post-baby body seems to defy the laws of physics! Four months after giving birth to daughter True, the 5-foot-10 star (pictured in Mexico on Aug. 14) credits breastfeeding, grueling six-day-a-week workouts and a low-carb diet for her weight loss of more than 30 pounds. And yet her success only amplifies the fact that her butt hasn’t budged an inch! That’s a sign her bodacious behind isn’t, well, hers, Michigan-based plastic surgeon Dr. Anthony Youn, who hasn’t treated Khloé, tells In Touch, adding that he suspects she’s had implants or fat grafting, in which fat is removed from one part of the body and injected in another. But Khloé, 34, has long insisted she gets her curves from the gym, not…