When u sit down for a long time. Then stand up real quick.?

You are describing clic symptoms of Orthostatic Hypotension. You body relaxes more when you are sitting and can maintain an adequate sitting blood pressure with less work as it doesn't have to pump so much blood straight up. When you stand the body switches gears and has to raise your blood pressure to keep blood flowing while standing. With the lower sitting effort to circulate the blood sometimes your brain suffers a brief lack of oxygen because the blood isn't pumping immediately with enough force to circulate enough oxygen for your body and brain. Usually after a minute or so the blood pressure makes it up and the symptoms go away. This can be very normal, especially if you just jump up and start running around which stresses your body even more trying to get the oxygen/blood to circulate. If you experience these symptoms you should sit back down for about a minute and try standing again. Be sure when you sit down that you leave your legs down (not up on a foot stool or on the couch). Other things that can contribute to this problem are dehydration or anemia. It sounds to me like yours is probably not due to anything abnormal as it is mild.