I'm exactly 29wks today an catch myself waking up in the middle of the night on my back or sometimes on my stomach. I get so worried as if I'm hurting my baby, cuz I knw your definitely not supposed to lay on your back!!
My Dr told me your body wakes itself up to tell you "hey you need to adjust!!" He said your body will automatically wake itself up long before any harm is done to your baby. Just readjust and go back to sleep. You can try propping a pillow behind your back so that you don't roll on your back. I'm 29 weeks today also And wake up all the time on my back.
I also go to sleep on side and wake up on back. Like that every pregnancy. Nothing to worry about as long as your not constantly on your back or spending to much time on it. It's normal to roll onto your back in your sleep though and not something we control. You may begin to notice though that you usually wake up shortly after rolling onto your back.