Why do i feel so weak and light-headed after donating blood? i donated blood 3 hours ago and only feel normal if i'm seated or lying down. when i get up and walk, my head hurts, i feel weak and i can feel my heart beating?

When blood is donated, you lose 470 ml of your blood volume. Thats 8% of your blood volume. It may result in low blood pressure, increased heart rate (blood pounding in your ears) and feeling weak and light-headed. Make sure that you are drinking non caffeinated fluids above your normal intake (2-2.5 liters for women). Take a few days to rest and eat a diet rich in iron containing foods.