Why have I been coughing phlegm?

I've had yellow/ green phlegm for about 3-4 months now. I have no symptums - fatigue, fever, weight loss, runny nose, etc. Nothing. I feal perfectly normal and healthy. I do not smoke and have never smoked. If I cough it is voluntary just to get the phlegm out. Also. last winter, I, and no one in my family got a cold or the flu all season.