Why every time i stand up, i feel like i'm going to faint?

Hypotension: You may have orthostatic hypotension. When you stand up, gravity pulls the blood in your veins downward and your blood pressure drops. Your heart rate increases to try to keep up with demand, however sometimes not fast enough. This can be caused by certain medications that lower blood pressure also. Visit your MD and they can measure bp as you stand. Stand up slowly and try to prevent falls.