I had episodes many years ago where if I stood up from laying down, sitting down or even lifted my head up in my bed to roll over I would feel faint. If I got up in the middle of the night to use the bathroom it would hit me and I would have to stop wherever I was and put my head between my legs. (I don't think I ever really fainted but saw stars and had tunnel vision etc) Now don't laugh, but there was an episode on the tv show House where this happened to a women...he finally pressed on her rib cage when she stood up and the fainting stopped. Can't remember what was wrong, however!
They never did figure out what was wrong with me and just chalked it up to low blood pressure. Mine was purely positional.