It literally could be dozens of different things. But, it sounds like the vet pretty quickly identified it as an acute injury. He's a little old to just be getting RA now, so that would make sense. Plus, arthritis probably would have been visible on the x-ray, and if the vet suspected something, there's a blood test that can be done.
Presuming your vet is competent, I wouldn't be too concerned. 12 isn't young, but it's not necessarily elderly either (for a small dog). I'd give it another month, and if it doesn't get better, you might want to seek a second opinion.