Just noticed more bad news for Sparky. If he was a younger dog I would suspect pano although I've never seen it in small dogs. But he isn't a growing pup so this is strange. I would probably have called the vet as soon as I noticed the change in the leg, but then I scare pretty easy when it comes to my dogs. Weird that arthritis would be coming on so suddenly and now in both legs....really has me wondering pomtzu. Your vet might decide to do x-rays when the time is up if he hasn't improved. I'll keep my fingers crossed that it disappears as fast as it came on.
Sorry about all the trouble and worry you're going through.