This used to happen to me a lot, too. I would be having a nightmare, then wake up and turn on the light. But then realize, I was still in the nightmare yet awake and couldn't move. Felt very suffocating. Then, I would awaken with a start, and get up and turn on TV or something to prove I was awake.
One time, I snapped awake, got up and ran to the door of my apartment, opened it, and fell into a black hole ,,,, still in the nightmare. Just thinking about it, makes me feel queasy.
It hasn't happend in years, though. Yay! Stress, makes a lot of sense as the cause.
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Edit: First time I saw A Nightmare on Elm Street, part 2 I think, where she is in the basement, running ........ wow, I could relate! Very few films like that will I watch, but totally relate to that series. mwah haha, my dark side ....