It sounds like you were close to conciously experiencing SP.

It is a state of not being able to move. You either hit it on the verge of falling asleep or wake up from a dream into it. I think it's more common to wake into SP though. I think you might have skimmed right past experiencing the paralysis and fell right into a dream. Which is what happens to me. The slipping and falling sensations happen just before you start really dreaming and if you can ride those out and stay aware you should find yourself lucid in a dream. Just think of it as a little roller coaster ride into lucidity and let it happen.