I can't be this much fitter in 2 weeks???? Right!?

Thanks everyone, in answer to some of your queries- I do take off the transmitter after each session and rinse out the strap. The weather hasn't really changed much over the last few weeks, mild but rainy (I am in England after all) My resting heart rate is about 54bpm, but also I am on beta blockers. It was around 64bmp about 2 months ago. I was reaching the 'zone' for the first 2 weeks with effort.I think we may be onto something with the over trained thing, I'm feeling exhausted today! I may just be pushing to hard to fast, I've not done 'real' exercise for a few years!!