I'm A 15 Year Old Drug-free Female And For The

I'm a 15 year old drug-free female and for the past few years I've had a issue with blacking out whenever I stood up sometimes. I went to my doctor but she said it was normal; however over times its gotten so bad that i can't even stretch while sitting with out causing a bout of : blurred vision, blacked out vision, shaking, uncontrollable body movements, which are usually followed by extreme light headedness before a massive headache. Any idea of what it could be?