Yesterday was a normal day, hubby took Chester for a run no problem. Chester is almost 2. Then in the evening Chester laid down and didn't want to get back up. It seemed he could barely stand on his back legs. He didn't even want to go outside or upstairs. At some point during the night he did go upstairs and sleep in our daughter's room. This morning, he didn't want to go down the stairs and when he finally went out side he just sat down with his butt kind of shifted to the right. He appears to be favoring his left back leg. If we touch or move it he doesn't complain. I know a trip to the vet is in order but any thoughts on what to look for?