Apparently Cosmo has an abcess in his leg. He is running a bit of a fever so he's being sent home with antibiotics. She shaved his leg to look at it but didn't see anything that needed to be lanced so I guess what we do is just give him the meds and wait for it to go away.

I'm relieved but still extremely cautious. I have had more pets than I care to mention go completely downhill after I'd been convinced by the vet that everything was going to be fine. (Not this specific vet, though.) I don't think I'll be able to relax completely until the lump on his leg goes away and he's back to acting like his old self.