What causes my lightheadedness?

i get lightheadedness spells frequently the doctors cant figur out what it is so if any1 can relate or no what it is plz let me no i have had ethis since i was 3 and it has been gettin worse since then im 15 now and when im walkin i get very lightheaded im often scared i will p out but it isnt always immediatly after i stand up i copuld have been up for a while when this comes on it doesnt last to long i have ped out 3times so i dont always p out but i have had 5 just the smorning and i have only been up for bout 2 and half hours when i get one its very difficult to tell where i am ive ran into things b4 and after the brown blocks that obstruct my vision goes away i have a headache for bout 10mins its not a bad headache but its there if any1 nos what this or can relate plz let me no