I, and even the vet, were very confused. I do not believe it is mouth rot, the inside of his lip looks fine. And it's just tense and awkward for him to move (although I'm not sure how the vet determined that..) I guess. I'm kind of tempted to try checking with a different vet, as I'm not sure that I can fully understand or agree with this conclusion... :/

Is there any other possible explanation as to why she wouldn't be drinking?