Hey Kelly - that is a really tough one. It wasn't that long ago mums were told to always sleep their bubs on their belly. There are some babies that have to be slept on their bellies or sides for medical type reasons - so its not that uncommon.
DD has pretty much always slept on her belly since about 6 months when I stopped wrapping her. It seems her natural sleeping position is on her tummy with her bum in the air - lol. I still put her down on her back, but let her roam the cot to find her own position. Your bub isn't far off doing that for herself, in fact - DD was pretty slow in terms of when she rolled and stuff.
I think you need to do what works and what you feel most comforatable with. If you can make bubs sleeping environment as safe as possible, then there isn't much else you can do. I think SIDS people will not be happy with you sleeping a bub on their belly, however, they can still guide you on how to make the rest of the sleeping environment safe and sound.
Best of luck - its a tough subject, but I'm all for doing what works best for you.
DD is 3yr 8 months - DS is 6 months