Kinda gross question!?

I don't smoke and I don't eat alot of dairy products. All year long I will cough and sometimes get alot of phlegm when I do. It's usually yellowish green. I cough it up alot more when I play my saxaphone. I'm 15 years old and am in pretty good health. I have no allergies. Why do I cough up phlegm so much? Its the worst at school. I do have sports induced asthma. I have no clue why I cough it up so much!