Becoming Breathless on Running

I've been running for many years now with little complications. I completed 2 half marathons in the autumn of 2012, quite satisfactorily. Towards the end of January I started to slow down and become breathless easily. I think it had something to do with the severe winter we have experienced this year. Last week I struggled to complete a 10K- personal worst, last year I achieved a personal best for the same course, that's 7 minutes down. I'm currently having blood tests with my GP, but I was wondering if anybody else has had similar experience, especially after this winter?