Dizziness

Hi, i am 16years old. Recently, i've always been feeling dizzy after exercise. Even the exercise wasnt a tired one or even after i keep walking around for some time, once i stop i'll feel very dizzy and feels like almost fainting. At the same time, i'll feel very weak and short of breath. And my body temperature will get very low. Since i'm a athlete in school, I always have training. Always feel very sick after dat, especially when i sit down. I remember there was few times i almost fainted during training cos short of breath. At night, i always cough and somtimes even feel like vomiting when coughing. There was once i felt so short of breath all of a sudden at night and felt like almost faint. Somtimes when i lie down on the bed at night trying to slp, i'll feel hard to breath as though something is blocking. I'll have to sit up to feel better. Is there any problem with me? I am really very worry about it. I hope to get help.