karynda,   Thanks for your question.    It's difficult to say with any certainty what the cause of your pain is.  It could be muscular, or a problem with your rib cage.  Though not typical, there is still a possibility this may represent heart disease.  Your smoking may or may not be a factor in this pain.   The safest thing to do is make an appointment to see your doctor in the next couple of days for an evaluation.  He/she will take a history, examine you and perform an electrocardiogram.  Then your physician will be a better position to determine what the cause might be.  He may request further testing before coming to any conclusion.   Best wishes and feel better.   Martin Cane, M.D.