I have had that problem for years. Again, the fitter I am, the worse it is. Often it is worse the day of a work out but it works on other days also. It is caused by a low blood pressure and low heart rate, you stand and the blood drains from the head. Since the brain isn't getting any blood, you get dizzy. It only takes a few seconds for the heart to respond and increase the blood flow. The dizziness goes away.