Even though you are a marginal smoker this is not helping the situation and could well be the cause. It's difficult to know. At any rate, you have something going on in there that should not be happening, and it could be chronic bronchitis or it could be a pneumonia (this does not necessarily make you feel profoundly sick, at least for a while). Although you're not one to go to the doc I would strongly suggest you make an exception, have a thorough exam including chest x-ray, and have this diagnosed and treated, because it will either become chronic bronchitis or pneumonia or, perhaps worst chronic obstructive pulmonary disease (COPD). You may have asthma, as well, and not realize it. It's pretty amazing how we adapt to slow, progressive compromise of body systems until something starts to really get severe. Since you're not having symptoms so far, you may well be able to dodge a bullet if you get yourself in to the doc ASAP and have this thing diagnosed and treated.
Hope this helps. Good luck to you. Please follow up here as needed.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist