So similar to my story! I am 5'8" 148 lb non drinker, never smoked and have always plared sports and worked out. I even work at the hospital and have sat down with many doctors about this and they dont have a clue. When I run I feel good and it may start some time later. It comes and goes, some days Better than others. I have checked my co2 thru a blood gas and my co2 and ph are normal. I really try to pay attention to what I eat or have done that day, on my bad days. I have notice when I eat a lot or eat certian things it gets worse. Have you linked to anything when you have bad days? My next step is to see a allergy doctor. Please lets keep tabs and compare to see what may trigger it and hopefully figure this out!! Thanks for your post.