Sian and feet in the clouds - I have exactly the same thing, fine for my last long run of 20m (aiming for a 3h45m marathon) on the 30th then got a nasty chesty cough. Had and a week and a half off and then a couple of short runs then 10m yesterday, the 10m absolutely killed me. I couldn't seem to catch my breath.
I went to a drop in GP yesterday and they prescribed me steroids and antibiotics for a viral infection.
Fingers crossed it will go in time!
Will try a couple of short runs this week, but trying to avoid the inevitable!