Maybe this doesn't relate to your situation, but for what it's worth-
When I was in my 20's, I fainted a few times for apparently no reason. Very scary, and very embarrassing. Turns out I was not eating enough, was too thin and I was anemic as well. (I was smoking back then too and that didn't help at all, for sure.) Once I gained a little weight, ate better food and took vitamins, the fainting went away never to return.