This is a predicament I found myself in many years ago. A history of smoking and asthma with ongoing symptoms and a feeling all is not well or as well as before. I trawled the web looking for answers and spoke to many doctors and respiratory therapists. Nobody had answers.
This post also relates to a previous post of mine where I pointed out that doctors don't always give you the full story. (See "there's no use in diagnosing a serious anxiety provoking disorder in a young non-smoker who btw suffers from health anxieties and who they believe should not have any overt symptoms")
Last night I found a post on the COPD international site that I'm convinced is either written by an expert patient, an RRT or a medic. It's sums up the approach to dealing with those with mild disease and maybe helpful to those of you seeking answers. This was posted as a response to a young smoker in his thirties with all the hallmarks and symptoms of early COPD.
"Sounds like mild COPD to me. Spirometry alone often does not capture emphysema in the younger cohorts. This is because doctors are trained to make the diagnosis when the FEV1/FVC is bellow 70% however this measure decreases with age and therefore someone in their early 30's should really have a much higher value, like 80-85%. This is coupled with the fact that spirometry alone only measures airflow limitation, one part of pulmonary mechanics, which is largely a factor of airway function - not the alveolar damage and hyperinflation of emphysema.
The way to check for emphysema and early COPD is with a full PFT including lung volumes and diffusion capacity in addition to a chest CT. Doctors are also taught that COPD is a disease of middle to late life when in reality the disease begins the moment one starts smoking or encountering other risk factors. Some people are more sensitive to COPD symptoms than others and may be more coherent to mild emphysema once they become aware that there is some underlying problem. Others, mostly those who are very sedentary, won't notice COPD or emphysema till it's much more advanced. Of course this is all confounded by the fact that you had per-existing asthma and it is easy for a doctor to blame your symptoms on that alone when you could be presenting with a more complex COPD-asthma overlap syndrome.
Then there's anxiety which by itself can create a sensation of breathlessness so that's another possible explanation your doctors could be considering. Chest x-rays are not always sensitive to early COPD and even if there were some mild hyperinflation many radiologists don't mention this because they know it is a very subjective interpretation until the disease becomes much more advanced or they lack prior imaging studies to compare to.
Then if it is COPD, there's really not much that is done to treat it in the mild stage other than smoking cessation and since you've already done that perhaps your doctors figure that . there's no use in diagnosing a serious anxiety provoking disorder in a young non-smoker who btw suffers from health anxieties and who they believe should not have any overt symptoms. If you do get a full PFT, ask for a copy of the results and pay close attention to the FRC, FEV1/FVC, FEF25-75, ERV, DLCO, and DLCO/VA. You'll have to do some digging to find out how to interpret these results but it's not too hard - some values are better lower than normal, some higher, ideally to be unambiguous everything should be exactly 100% predicted, but there is a particular pattern COPD follows and the differences may be subtle in mild emphysema. If you can't get a CT or at least a full PFT maybe try for a Cardiopulmonary exercise test. Even the most subtle underlying lung or heart disease will be demonstrable on CPET and should then provoke further diagnostics to explain such.
If these results still don't explain things I would wonder about GERD or maybe an extrathoracic airway problem such as vocal cord dysfunction since you mention chest tightness and trouble breathing in, respectively.
COPD'ers have trouble breathing in but most of there trouble is breathing out due to decreased pulmonary elastic recoil and what's referred to as air stacking."