everytime i stand up, or start walking, my vision gets like fuzzy then goes to black and i cant see and then i get light headed but it goes away after a couple of seconds. what is this? its not from standing up too fast, ive tested that theory, also i have an eating disorder, but even when i eat and am healthy it still happens, the doctors said i was fine a little while ago, nothing was wrong. this has been happening for a while now