Once they can roll over on their own, they have the reflex ability to know if they stop breathing or not. She will be fine if you just let her sleep how she wants. Still put her in her crib on her back, and then let her be until the morning. I know it is nerve wracking, but she will be fine. She must prefer sleeping on her tummy and putting her arm over her mouth. You could go in every hour or so and turn her over, but what would be the point. You would get less sleep, and she will continue to roll over again. Also, then you are waking her up and her body will get used to that and she will get up constantly through the night until you break the habit. So, if you feel more comfortable, you can always call your pediatrician's office and discuss it with them. Good luck!