hmm idk bc for some reason lately i always start out on my side but when i wake up in the middle of the night i'm ALWAYS on my back bc it's so much more comfortable and i've never been the person to sleep on my back, i think that is a ? for your ob and maybe i should ask mine too lol
i am not really sure if you would know or not, but i have heard that you could start to feel sick and dizzy from it. I am not able to lay on my back for very long b/c it causes contractions, but i would just check with your dr and see what they have to say.
I spoke to my doctor about this earlier on in the pregnancy because I was concerned with the sleeping positions. And she informed me that the left side is the best (for obvious reasons), but that you sleep how you can to get the rest you need. Yes, after a certain point back sleeping isn't good, but like she said, if you sleep on your back, you will wake up and turn when it's not comfortable anymore. And that's what I do. I've go to where now I can't sleep on my back because it causes the worst back pain. My best sleeping position is half side and half stomach or just on my right side.
I've read the same as kylesMom09 stated that you'd feel sick and/or dizzy if you shouldn't be on your back. Personally, I usually can't stay on my back for long without becoming short of breath, but sometimes like you I wake up on my back and I'm breathing just fine. I think your body would know what's best for you. Left side is best, but unless it physically hurts you to sleep other ways I think you'll be alright.