I've had the exact experience. In my case it was dehydration.I had gone on a trip, didn't drink a lot of water, I waas in meetings a lot. I did a lot of flying which is dehydrating. Then the next morning I got in the shower. As the hot water dilated my blood volume a black curtain just began coming down over my eyes. It scared me so I jump out of the shower and the adrenalin got my blood pumping so the vision came back. If your daughter was not dehydrated when her bllod tests were done, nothing would show as abnormal. Think if she drinks enough water, does anything that would require more. Was she bathing or showering before any of these incidents? Though she didn't faint, many people that do, get this feeling too. It isn't usually a serious condition and happens more in young people because their blood pressure is lower. It may also happen when they have been sitting or lying a while and get up suddenly.