Coughing up mucus for weeks.?

I've been coughing up mucus for weeks now, and some days I'll have a really sore throat and stuffy nose, like I'm sick. But then the next day my throat will be fine, and I'll be back to just coughing. I do smoke occasionally, so I'm thinking this is probably bronchitis.. or maybe walking pneumonia? It should be noted that I have a weak immune system and get sick a lot. The mucus is usually thick and yellow, and it's like I have an unlimited supply in my chest. It's sometimes hard to breath. Thoughts?