Confused.

If a baby rolls on their tummy to sleep, but they can't roll back, what do you do? My baby's 7 months and I've never seen her roll tummy to back before except once when she was 3 months old and never again. However when she sleeps she presses her face down into the mattress, not to the side, I mean literally face down. It scares me because I don't see her nose and can't tell if she's breathing and I don't know if she's able to roll back if she can't breathe. What do I do. It seems she is most comfortable face down like this, although sometimes I see her lift her head almost like she's coming up for air. If I roll her back she immediately rolls to her tummy to be face down again and falls asleep like that. I've been up all night.