My vision often goes black at times, usually when standing, followed by immediate dizziness and becoming extremely light headed, almost to the extent of losing my balance. After a quick 30 second to 1 minute of this occurring I feel somewhat back to normal and carry on with my day. Tonight I checked my blood pressure with my parents reader and my heart rate was 69 with a pressure reading of 103/66. I feel fine besides this occasional disruption at least once every 2 or 3 days. I'm 20 yrs old, in shape, healthy [besides whatever this is], and completely confused as to what this could be caused by. It's been going on for about 3 years now. Got any insight on this issue at hand?