rolling over at night

My baby who normally sleeps on her back is rolling over to her tumy at night now (5 1/2 mon). She wakes up fussing and up on her arms and cannot seem to roll back on her back. She initally did roll over from front to back in playtime but not regularly. I have been going in and turning her to her back again and it seems like she rolls back immediately. We were up several times last night doing this and I hate to leave her up on her amrs trying to roll back crying. Any suggestions?