Here is a good way to tell if you have orthostatic intolerance. You are walking around the supermarket for fifteen minutes and you are doing alright. The act of walking helps to circulate blood to your brain by the muscle contractions in your legs. Then you stand at the check-out counter and the person in front of you has lots of coupons. You stand still for ten minutes and then you feel that you have to lie down. The standing still causes blood to pool in the legs and less gets to the brain. This is orthostatic intolerance.
Dr. Bell on Understanding Orthostatic Intolerance in Chronic Fatigue Syndrome and Fibromyalgia