I just found this forum for the first time and responded to another post - I was diagnosed with a rare lung disease called LAM (or Lymphangioleiomyomatosis) at age 25. This disease affects women almost exclusively and symptoms generally appear in women of child-bearing age. When I found out I had this disease I had never heard of, I was running 3-4 miles several days a week and had encountered some exercise-induced asthma in high school but nothing else. I had recurring episodes where it felt as if I pulled a muscle in my back/chest which made it difficult and painful to breathe. After several months, I randomly caught bronchitis and went to the PCP, got a chest x-ray and determined I had a collapsed. lung. I know these are not in-line with your symptoms exactly, but one test I could suggest would be a chest CT if you have not yet had one. My lung repeatedly has small collapses that make me short of breath but the collapses do not always appear on a chest x-ray; however, they do show up on CT. The smooth muscle-cell growth also does not appear on my chest x-rays.
If other women of child-bearing age have experienced chronic shortness of breath, chronic cough, pneumothorax (aka lung collapses), chest pain and fatigue that have not been explained, I would encourage checking http://www.thelamfoundation.org/ for additional information and mentioning the disease to your doctor if it seems like a possible diagnosis. Symptoms are often similar to asthma, emphysema and/or chronic bronchitis and it is believed that hundreds of thousands of women may currently be undiagnosed or misdiagnosed.
I'm going through something very similar. I know how it feels. I've been experiencing shortness of breath for the last 2 months. I've had every test done on me like the spirometry. They came back normal but I'm slighly below average on the breathing test. I've never had breathing problems in the past and inhalers do not work for my breathing problems. However, recently I got a scan of my sinuses. It appeared I had a chronic infection so I suspect that is the reason. If you havnt already get a ct scan of your sinuses also it could be that. Also going to a allergist/ asthma specialist has helped me find some relief. Also it could be GERD. Hope that helps good luck.