If you're losing fat then you're going to...well, lose fat. Guess what was on top of the muscle in your butt?
If you want exercises to target the gluteus area, here:
Hip Exercise Menu
Lunges and squats! And as an added bonus, that extra muscle you build down there will help you burn more calories to help with fat loss all over your body.
If you're losing fat, you can't decide where it comes from..., but as others have said, concentrate on doing more lunges and squats, it will help build those muscles in your butt.