If your vet is still open (this being Saturday some only do a half day), give a call and see if you can run him back in and not pay for a second office visit. Usually they are good about this stuff. Even say you don't know if it is just a reaction to the shot, and you need to have it checked. They shouldn't charge you for that!

Yup, tough watching our loved ones age - furkids, pawrents, I got it both ways.