When I would walk my vision sometimes would go black. I always thought that it was because I was standing up too fast. But the last few months I can be walking and my vision will go black and my entire body will start to shake and after I feel funny for a few minutes and lately it has been happening more often. and it is not always right when I get up sometimes I can be walking for a little bit and it will do it to me. I don't know why it is doing it or what it is?