If your cardiologist says that your heart is normal and pain is not because of cardiac cause then another common cause of chest pain
and upper back pain
one is gastritis
i.e. increased acid secretion in your stomach which causes chest discomfort
second is musculoskeletal like degenerative disc disease
which is more seen in smokers .
ithink you should go to orthopedic surgeon and get your upper back ( thoracic spine) x ray done.
neverthless remember one thing that though ,today if dont have cardiac cause of chest pain your heart is definately at risk if you continue smoking