Hey buffaloguy. I have had a similar experience a couple times where I would slowly fall on my stomach during a lucid dream, then wake up on my stomach. It sounds to me like you could be becoming lucid so close to the end of your REM cycle, so you began to feel your actual body (which for you is on your back because that's the position you sleep in).

However if you catch it in time, you can still extend the dream by remaining calm, and even doing very little, as Patrick pointed out. Sometimes less is more. Question: when you rub your hands together, are you also looking at them? I find that helps, looking at my hands, legs, feet. And in your case, also not looking up until you're comfortable with the stabilization.

One final note, if you are waking up from these lucids and it's still early, you could try DEILDing and reentering the dream from a lucid state, and extend it that way. Good luck!