In reply to mike lawrence?:
This is the naughty way, but if you suspect asthma and you know someone who has a standard blue inhaler, borrow it (obviously sanitise very carefully before and after at present or borrow one that's not got much left and bin it afterwards!), inhale 4 puffs or thereabouts and go for a run. If it works, then that's your answer.
I had an experience very similar to yours, though when I "run out" I always end up heavy breathing - does that happen to you? It turned out it was asthma, most likely post-viral, but the investigations had reached stalemate (i.e. we know it's happening but not why) so me taking that action allowed it to be moved on to a conclusion - I told the doctor of my experience and they booked in all the asthma related tests.
Asthma wasn't originally suspected for me because I don't have proper asthma attacks - it's just shortness of breath. We went mostly down the heart route which took some time and found nothing at all other than a quite low resting pulse rate which is not unusual for runners.
By the way, if it is asthma and you end up getting a preventer, don't be afraid to ask to try different ones, they have different side effects and different levels of effect overall. Most notably I find that anything involving beclamethasone (e.g. Fostair - but also the likes of Beconase) makes me depressed - basically I end up tearful for no reason at all about an hour after taking it - it was bizarre and quite unpleasant until I worked out what was causing it.
Post edited at 14:07