I know now what's causing it: inflammatory cytokines. I'm reasonably certain of that.
The pulmonologist found no problems with my lungs or heart. I'm still pending an allergy evaluation (next week), but I doubt that will find anything.
Here's what I know:
Things that make it worse:
Exertion - Even walking now seems to trigger it. A flight of stairs: definitely.
Coffee and other stimulants - The strange thing is, even decaffeinated coffee triggers it. I believe that even the small amount of caffeine in decaf coffee is sufficient to make it worse. Much worse actually.
My car - This is the one that doesn't quite fit with the others. My first thought was, maybe I'm reacting to something in my car, particularly in the ventilation system. Other times I've thought, maybe it's simply because every time I'm in my car, my presence there was preceded by exertion: gathering my things and walking to the car. But I don't think that's the explanation.
Things that make it better:
Nothing, except avoiding the things that make it worse. I've tried Monolukast, inhalers, tea, mold and chemical avoidance, diet, anti-inflammatory drugs, Azelastine, and Flonase. None of that makes a difference.
My conclusion is: This has to be inflammation triggered by cytokines. We've ruled out nearly everything else. Exertion and stimulants are two factors known to trigger an inflammatory response in people with ME/CFS. The car as a trigger is less easy to explain, but I feel there may be an environmental trigger to the inflammation as well.
I've been ill with ME/CFS for 4.5 years, generally the time frame when, as studies have shown, a person's cytokine profile changes.
Shortness of breath was a major symptom of my acute phase in 2011, but then it went away for the most part, for the next 3 years. During that time, it was only sporadic, fairly rare, and not too severe when it did come. It was not accompanied by PND like now. I enjoyed a higher than average exertion tolerance compared to some other patients during that period. Basically, I was able to do most day-to-day activities without fear of triggering a major crash. (I dealt with a host of other problems, but hair-trigger SOB wasn't one of them.) I wasn't like a normal person who could go running or play basketball, but I could climb a flight of stairs and feel OK generally. That may be changing now.
Some people say that SOB in ME/CFS is not a matter of lung function or blood oxygen levels (my blood oxygen levels are fine) but rather the body's inability to use the oxygen efficiently. I don't think that's the problem in my case because the SOB always gets better and worse in lock step with PND. It makes more sense to me that inflammatory cytokines would be causing lung inflammation and at the same time, causing my sinuses to produce mucous. The inefficient use of oxygen theory doesn't explain the PND.