Poor Kaydee

Poor Kaydee was running round a field when she suddenly came to me tail between her legs and very distressed. She was skipping on one back leg, after much examination we wondered if she had been stung by a bee. However over the weekend she began to look worse, hopping on both back legs Saturday and looking lame on three legs by Sunday. I took her to the vet today and he could not find anything, nor can I. No heat or swelling anywhere, no pain response to manipulation, no loss of range of movement. I was looking forward to a week of walking in the Lake District hills and was due to leave Friday..... as she doesn't want to walk anywhere right now I guess I had better cancel! It breaks my heart to see her so miserable. And so frustrating that I don't know what's wrong and don't know how to help her.