Hi, I'm a 21 year old girl, I weigh 122 lbs, and I am 5'3". Since I can remember when I stand up after sitting for a while, or even just randomly get up not after a long period of time I become dizzy. Many times I simply stagger through it and hold on to things as i go so I don't fall down. However, every-once in a while my vision will go either blinding white or totally black. During those times I usually plop down on the ground and wait for a few seconds to a few minutes for the spell to pass. Other times i become so dizzy that i have to sit down even though my vision hasn't gone out. Why does my vision sometimes go white and other times go black? And also is this very serious or simply just another case of low blood pressure?