Hi, so this has happened twice to me, once yesterday and once in January. I start to feel very nauseous so I start to move from wherever I am, and as soon as I do I completely loose my vision, everything is completely black. I can still hear and I m still aware of where I am and whats going on. I ll also be sweating and still nauseous and I either sit down on my own or end up falling down, its very scary and I don t know what causes it because I was doing completely different things both times.