What Causes Vomiting Up Of Saliva And Mucus In Person With COPD?

Dear Doctor I have been recently diagnosed with COPD. I am also spitting a lot saliva and mucus, I am going around carrying a tumbler to spit into. I went for my review today, and my doctor told me that my chest has not improved, he however prescribed Celestamine tablets (30). this was my second consultation. I am expected to be back after a week, where, if my chest has not improved, I will be referred to a specialist. I am worried, what is causing the spit?