i am 17 years old, pretty thin for my age, and used to be in real good health, but now i dont know whats wrong with me. i get headaches all the time.. i feel lightheaded a lot, and whenever i walk up or down a flight of stairs i get short of breath, also if im walking somewere and talking to someone at the same time i always have to stop in the middle of a sentence to catch my breath. sometimes i can feel my heart pounding in chest. my pulse is always on the high side, even if im just resting. sometimes my heart just feels like its beating really hard. im always tired. i went to the doctor for a check up over the summer and they said i just need to be more active, but i used to be, but now i cant because whenever i run or exercise, my heart pounds really hard and fast, i get lightheaded, and get headaches and get short of breath. i really dont think thats it. what else could be wrong with me. i know people can say doctors can tell you whats wrong with you, but i really dont think he's right this time, i just dont feel right..