When I stand up I blackout. This happens so often that I often brace myself or crouch down. I could be standing outside and walk inside and this happens. My eyes are open and everything goes black. Afterwards, my pupils are dilated, I feel week and dizzy. I don t really do any exercise anymore because I feel tired all the time and dizzy when standing. I am 20 years old 5 4 and 100 pounds. I have trouble gaining weight and eat quite a bit.