You mentioned birth control which immediately brought to mind "water retention" and "fat." No, not calling you fat.
You've probably still got the muscle under there unless you've changed your activies quite a bit. BC can cause any number of problems (being hormones, obviously) and one of MY biggest problems was water retention. Make sure you're drinking plenty of water. Also, if fat has started accumulating there (again, another lovely hormone issue), the only way to really take care of it is to drop your overall body fat.
BC is a giant pain in the ass.