Sudden blindness when standing up.?

In the last few months, every here and then when I stand up I get a sudden "Blindness". What I mean by blindness is that everything goes white on the outside of my eyes then travels inwards until it goes completely white. I feel top heavy at times and almost fall over. The worst case of it I had was it lasted for about 20 seconds and I also lost my hearing during the time. Does anyone know what causes this?