I'm not sure this is related to migraine. There's a condition called "postural orthostatic intolerance" that sounds very much like what you are describing.
With POTS - postural orthostatic tachycardia (tachycardia means rapid heart beat) - on assuming upright posture, there is an excessive increase in heart rate. Most patients experience light-headedness, but 30% have blackouts. The changes in blood flow could also cause the pain you feel in the head.
I would read up on it and if this sounds likely, then talk with your doctor. Here are some linkswww.dysautonomiainternational.org/page.php?id=30www.potsuk.org/gp_guide