Our expert says:
Allergies come to mind, possibly caused by change in seasons, maybe a new pollutant in the area? It's worth going for an allergen test, where they check for the common ones. But it is very peculiar to suddenly be out of breath. Had you taken a long break from running and then started up again, then it would be quite normal. And you've done all the right things in terms of checking for asthma and heart problems and lung x-rays etc. So that should be a relief, though not exactly comforting that you don't know what the problem is. Just on the note of asthma, I am assuming you had it tested for, because it can develop later, often as a response to some allergen. And there is also a thing called Exercise induced asthma, which is only picked up if they do a very specific test for it. So that's something worth thinking about too.
I hope the allergy issue is the answer - what you might try is to exercise with your mouth and nose covered and see if that helps - in otherwords, warm the air as you breathe in. Not practical, but it does give useful information to the doctors.
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