I've had this exact same issue for about 2 years. Once, it was bad enough that I lost consciousness for about 5 seconds. For me, it all started when I dropped 50 lbs and got closer to the weight I should be. I never had issues when I was heavy, but now that I'm in better shape, I get dizzy very often when I stand. It seems to be more severe after I've done some exercise. I've been to the doctor, and have had a series of tests run (stress test, 24hr EKG, tilt table, neurological tests, etc) and the general consensus is that there's some issue with the way my heart adjusts for me changing from a seated to a standing position. During this transition, gravity will cause blood to run to the feet, and the heart needs to pump more strongly to get adequate blood to the brain. This generally happens so quickly that you don't notice it, but for me there seems to be a delay of a few seconds that causes dizziness. Both the cardiologist and neurologist agree that it's nothing that I need to worry about unless the condition worsens, and I should continue to monitor it and just be really careful when standing. My advice, though, would be to check it out with your doctor. Just because this is what's going on with me doesn't mean it's the same for you.