Can anyone tell the cause of Mucus Cough?

I am 15 year old girl and I eat well, but for past year I have been coughing up mucus constantly. Sometimes it is really thick and yellow and sometimes it’s really string. I can feel it in my throat all day long. Please tell me what could be the cause of mucus cough? Is it something like cystic fibrosis? Please answer back as soon as possible. Thank you.