Retrosternal pains can be due to variety of reasons -
Gastro esophageal reflux ( Indigestion
Pulmonary or the lesions in the lungs can lead to pleuritis
It is very hard to say what exactly is the cause in your case, as you smoke ,chances of indigestion or gastro esophageal reflux are very high.
I would advise you to consult a doctor to find out the exact cause.