Frequently during transition I do get the acceleration feeling - like flying head first at incredible speeds. I call this headrush. It's a common sensation when entering lucid dream. Lately, I started having this headrush during my regular dreams, when in a dream I decide to lay down, or bow my head in order to go to sleep and wake up lucid. At these moments, I sometimes feel very sleepy, almost falling over. When I wake up from these dreams IWL, sometimes I can't tell, if these sensations were followed by a lucid dream or not.

Maybe you are going through this sensation and entering, or almost entering a lucid dream, just not remembering it. That's my best guess. Could be something entirely different, though. Happy dreams