Mr S., I suspect that you pushed your body too fast, riding so many miles so soon after starting to cycle. I don't think your muscles and joints and ligaments had a chance to strengthen and build up to what was being asked of them. Now something is slightly damaged, and you'll need to back down and work gently for a while and build up v-e-r-y slowly to where you were before. Just my own humble opinion as I am no expert.