My baby did the same- still sometimes does- and generally started fussing or crying soon after. Despite being strong enough to roll on his back or push himself up, he still starts crying when he rolls onto his stomach sometimes. No sleep sack, though, not sure how that changes things.
Can you hold your baby? Kiddo falls asleep with my arm around him. Most of the time that prevents movement, unless it's too hot- then kiddo will roll away (it has been the last few nights).
If you really aren't comfortable, can you find another option?
He falls asleep on my lap nursing and then I lay him down beside me...he transfers fine surprisingly. He doesn't cry at all when he rolls on his stomach...he seems to love it. Two nights in a row and he's slept more than he has in a month! I guess he's ok....??
Reading this may help ease your mind: http://www.parenting.com/article/ask-dr-sears-co-sleeping-a-sids-danger
I've seen a lot of research about bed sharing reducing the risk of SIDS because the parents, esp. nursing parents, are tuned into the baby better, so if something did go wrong- you'd be aware and respond. I imagine that you're aware of your baby's first fusses. You can find ways to stop your baby from rolling if it's really important to you, though. Consider finding a position where you're holding your baby so that he can't roll as easily.
I don't know for sure, but I feel like the risk of SIDS with belly-sleeping is lessened once your baby is able to hold his head up and push his body up while on his stomach. If your baby can do those things, then I wouldn't be as concerned. Again, I'm not positive on that one.
That's what I've been reading a lot...that if they can roll over themselves they should be fine. Last night was night 3 of tummy sleeping and he slept 10-5! I ditched the sleep sack and just dressed him a bit warmer. I scoot down a bit and over so he has lots of room. I guess I'll continue to go with it!