I have had a few of these dreams, and I know exactly what you mean by the strong feeling of oppression and helplessness of not being able to move a muscle. It's been awhile, but as I recall in each case where I had this paralysis, I was dreaming THAT I WAS SLEEPING (for example, I dreamed that I walk into a motel room, crawled in bed and went to sleep). And furthermore, this paralysis occurred when I was waking up in the sleep in my dream (that is, not waking up from my actual sleep!). It makes me wonder. Perhaps there is some kind of special circuitry in the brain that specifically addresses your mental state at the moment you fall asleep, such that when you wake up this mental state is reestablished through this circuitry (or might I say "activation memory"). Perhaps the same circuitry is activated or invoke twice in these unusual cases, but because you are in effect still dreaming your body is indeed paralyzed but another part of your brain thinks that it shouldn't be! Pretty wild, huh? :-)