Cough and mild chest pain. Worrisome?

Sir, I have been having cough for the last 20-25 days, which is sometimes wet and sometimes dry. I also caught cold twice between this time interval. initially it was only cough with mucus, lately, i have been feeling a mild sensation of pain in my chest(mostly in left side, sometimes in right too), and chest feels heavy most of the time. mucus: color varies. sometimes dark and thick, sometimes light and yellowish. No blood. At present, the cold is mostly gone and mucus discharge has been largely reduced. smoking history: 4 years (on an average 3-4 cigarette per day), haven't smoked for the last 6-7 days, since the conditioned worsened. No family history in such conditions. No fever. Kindly suggest what could be possibly wrong? is it just a routine chest infection or something far more worrisome? please help!!