so, i'm glad that you posted this, as i have the same issue. i have been diagnosed with vasovagal syncope (VVS). VVS is a neurological and cardiovascular disorder, wherein your vascular system cannot adequately respond to changes from various variables. the response, which most likely ends in a drop in heart rate and blood pressure (hypotension), and possibly fainting. for me, i mostly get extremely lightheaded, tunnel vision until everything goes black, and i also "see stars." several times, i have indeed fainted. triggers of a vasovagal episode include fatigue, stress, hunger, high temperature, and most commonly changes in altitude (e.g. sitting or standing up), among others. however, i've noticed that when i lift my arms above my heart, i get a similar response.
the most common way to be diagnosed with VVS is a tilt table test. you can read more about it and VVS, in general, at this wikipedia article: http://en.wikipedia.org/wiki/Vasovagal
to control this, i have been taking an SSRI antidepressant, celexa to be specific, for the past six months, and it seems to have cut down the frequency of episodes. however, make sure you drink plenty of water, increase your salt intake (to retain fluid in your bloodstream, just in case you are also hypovolemic [low blood volume]), and to get plenty of rest before you exercise. but, i'd go see your doctor to get a referral to a cardiologist so you can be adequately tested. one thing you can do preliminarily, is to lay down flat at home for 5 minutes (to let your blood pressure normalize), take your pulse (and blood pressure, if you have a cuff and know how to do it), then stand up quickly, and take your pulse (and pressure) again. you're looking for a drop in your pulse and pressure. your physician can also do this in his/her office just to get an idea of whether you may have VVS, before getting a tilt table test.
hope this helps!