Feeling of black out without actually passing out?

Okay, so you know when you rub your eyes and your eyesight like slowly comes back starting in the center? Well that keeps happening to me in reverse. I'll just be standing there and my sight goes away like that and I get a bit dizzy. It doesn't last long, and I can shake it...sorta. It happens several times a day, and with no ryme or reason to it. It's not just like when you stand up too quickly - though sometimes it is. I'm a 15 year old girl, healthy weight, no previous health conditions, if any of that helps.