It's difficult even for a doctor or other medical professional to tell what is causing chest pain, cause, any part of the chest can be the cause of the pain including the heart, lungs, esophagus, muscle, bone, and skin.
Many users have written about same experience, having chest pain after quiting smoking.
These chest pains can be due to stress and anxiety felt in a period of withdrawal.
Also, it could be due to some pulmonary infections.
Do you have some other symptoms along with chest pain?