what these guys have been saying is true, getting up quickly can cause a headrush, coupled with the low blood pressure already from the weed already would definitely cause those symptoms. also, the act of smoking alone can cause headrushes because instead of your body inhaling oxygen, it's inhaling smoke so your brain might not be getting enuff oxygen= headrush. a few friends and i were smoking out of a third lung once when one of them blacked out from it. i'd say its just from the lack of oxygen/low bp.