What is wrong with me?

I am 17 years old, exercise regularly, I would say I have a fairly healthy diet, I'm 5ft 3 and weigh about 115 pounds (8 stone something) ... Although I seem to be tired all the time and have no energy to do anything.. When I get up I have breakfast which is usually something like cereal.. But then like a hour later I can feel myself getting hungry again and like a little light headed and there has been a few times that I have left it a few more hours and fainted :/ I find myself constantly hungry and always wanting to snack on little things... Not like chocolates and cakes or whatever but can be things like fruit.. Just wondering if this is normal for my age or whatever or is there something I should be doing differently...? Thanks..