To be honest, it sounds like SP to me. I'd think if you fell out of bed for real it would wake you up, or at least you'd have bruises or be able to feel the residual pain after waking up.

1st question - when you're sleeping, how dark is your room? Is there enough light that you'd be able to see the ceiling?


2nd question - when you wake up for real, are your sheets all tangled around you or placed weird, as if you re-arranged them while asleep or half-asleep? Or are they still neatly placed?


You could try putting something on the wall or ceiling where you'd be able to see it (if this happens frequently) like a piece of masking tape or something, and see if it's there next time this happens.