Though the present pain is unlikely to be due to any serious lung condition but your airways may have become hypersensitive because of all the smoking
hence you are feeling the pain while breathing
. I suggest that you quit smoking as smoking is responsible for many a diseases of lungs and other organs. The pain could also be of muscular nature or it could be because of reflux disease causing oesophagitis. Consult a physician who can then advise you accordingly.