Bellabub, I often wake on my back - don't stress. I'm getting bigger - 32 weeks now and I'm still comfy enough on my back (well, as comfy as you can be in any position with a big, heavy tummy!). I use a pillow like Alan recommended, but it doesn't always stop me from being on my back. Like Blackbird said, sleeping is getting harder now, so probably the last thing you want to do is worry about it & lose even more sleep over it. I've been told that if you feel ok - not light-headed or anything, your baby should be just fine.
Take care, and keep enjoying your pregnancy!