What Could Dizziness On Exertion Or Standing Up Fast Indicate?

I am a 14 year old girl, i am around 5'7 5'8 and weight around 115 pounds, i am extremly heatlthy. Ever since I was little I remember getting dizzy, It usally happens when I stand up to fast or I work really hard in sports (such as ice hockey, running, weight lifting, and lacrosse) I don't want to stop trying my hardest in sports, especailly when I have a chance to make Varsity my first year of playing high school sports! At first I get the feeling like i'm going to faint, then, everything goes black, and I can't then remeber what I was doing, I am consensus through the whole thing, and I sometimes fall over. I drink water, and my parents think its because I dont eat enough. I eat enough I always finish my food and everything. And when I feel dizzy i usally eat after, it does nothing to make the feeling go away.