Hi, could be a number of things but just wanted to add that a similar thing happened to me a few months ago and after a lot of doctors visits i was finally diagnosed with glandular fever... most likely you're just tired or burned out but get to a doctors and get checked out. In the mean time, rest over christmas?
Thanks again for the advice, I ended up doing 18 miles and have now cut down to rest my legs before the big day, can't believe it will be over a week today, very nervous now!