This condition is called Orthostatic Hypotension. This is where when a person goes from a laying to standing or a sitting/squatting to standing position, the blood pools in the legs and makes the heart work harder to bring the blood up and back to circulating through the body. This can be caused by many conditions such as dehydration, anemia, heart problems, high doses of blood pressure medications, and diabetes among others. If this has just started happening suddenly, you should see your doctor soon to rule out any of the above conditions. If this has been going on for a while, make sure to drink 8-10 glasses of water a day and walk a mile at least 3 times a day for conditioning. Try this for a week or so and if the problem still persists you should see your doctor for a work up. I have added a link to help with more information.
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