My dog is limping and I dont know why?

I have a 2yr old chinese shar-pei. I got home from work on monday and he was fine...jumping around and playing with the other dog...an english bulldog. I left the house to go get sum groceries and when i returned, the shar-pei's left hind leg was bent up and he was limping. i examined him...checked his paw for cuts or objects..nothing. I moved his leg around and lightly stretched it..nothing. he never once yelped in pain, cried or moved away. I took to vet on tuesday afternoon. he did have some swelling in the HOCK and was put on steroids and antibiotics. today is fri afternoon and he is still limping, can it be broken and the vet missed it? other than that he is eating normally, going potty normal, still manages to run around on 3 legs. im just SO worried as to what it can be. Any ideas, i would really appreciate it!!!!