Danielle, Kaetlyn does the same- she started with sleeping on the side, now its on her stomache. Every night I worry about it. Sids reccomends 'putting them in the cot on their backs to sleep' however, after much research, there is really nothing that can be done.
So I place her on her back to go to sleep, then when I go to bed, I turn her back over from her stomach, but before I even leave the room every night without fail, she is on her front again.