Can you please help me with my Beagle?

I have a 5 year old male Beagle and and about 5 days ago he started favoring his right front leg. I have felt all around on his left leg to see what could be possibly causing him not to use it, and I couldn't find anything. There aren't any cuts or bruises, and he isn't showing any signs of being in pain at all. We took him to the vet and they couldn't find anything wrong with him, but gave him a cortisol shot and said that should help but it hasn't. He is still not using his leg. He constantly has it tucked up beside him, even when he is sleeping. I've also noticed that it seems to be twitching, or having some kind of spasm. This is also going on while he is asleep. We didn't get an xray while at the vet, I think I'm going back to get one, but this is really strange. Does anyone know what could be going on? I am getting worried!