This used to happen to me a lot when I was growing up. It wasn't a problem usually, but one morning I ended up on the floor of my closet. Then once I got dehydrated and disoriented, losing vision for a few minutes. My parents took me to have tests run, and they never found any problem, said I was perfectly healthy. It happened less and less as I got older. Now very rarely, and never to the same extent. I was always pretty skinny as a kid and never did much in the way of cardio, so I think that may have had something to do with it.
_________________
"I was made to love magic, all its wonder to know, but you all lost that magic many many years ago."
N Drake