If she is more comfortable in sleeping on her stomach, let it be. May be she gets some blocked nose on her back and she is able to breathe better in this position. There is no ideal posture for sleep, in whatever position the child is comfortable is fine. If you are worried that by lying on her stomach she is rubbing her genitalia and gets pleasure, even this is normal and should be ignored. This will settle over a period of time as it may happen in some small babies. Just make sure she does not have urine infection.