I am a 20 year old female - 5 7 , 120lbs. For several years now, whenever I stand up from a sitting or, especially, a reclined position, my vision goes dark, I feel light headed, and my arms and legs feel very heavy. It used to be that I could always just go about my business and after a few seconds I d feel totally better. I went to the doctor about this because, for a month or so about 2 years ago, I was randomly passing out during these spells. They did an ultrasound on my heart and gave me medication to raise my blood pressure. The fainting then stopped and the condition went back to my normal level. But over the last 3 weeks or so, when I go from lying down to standing up, my vision goes almost totally black and I feel like I can barely move my arms or legs because they re so heavy. I feel so faint these days, that I d say about 60% of the time (lying down - standing) I wind up falling to my knees and putting my head on the ground so that I don t pass out. After a few (15-45) seconds, I m able to stand up and carry on. What could be causing this?