Anorexic guinea pig

Question
Please help! Vet told me he doesn't know what is it. My male guinea pig who is 1.5 years old started acting weirdly a week ago. At first, he stopped eating his pellets. Then he stopped eating hay and started being lethargic. I took him to the vet. Vet listened to his lungs and said he hears something, so he thought it was URI. (he had very bad pneumonia in August, so we thought it was back)He gave him some antibiotics and vit C. And the next three days he was getting antibiotics and vit C, but he was eating less and less( no hay or pellets at all, only some veggies). And then we stopped with therapy because vet sad it isn't getting any results. Through weekend i had to hand feed him because he wasn't eating at all. Yesterday he got diarrhea. Vet said it was because he isn't eating, so he gave him something for apetite. He openly told me he doesn't know what it is. He suggested we could do teeth trimming, but he thinks that isn't the problem(he examined his teeth and he didn't saw anything suspicious). If he doesn't get better by tommorow, we will do that. I am a little bit scared because of anasthesia, but we don't have much choice. And what is ironic, my piggie looks like he wants to eat. He calls me, sniff around, but when i give it to him, he walks away(sometimes he just sniffs it, sometimes he trys it and then goes away. He is lethargic, but when i call him, or when we are at vet's, he runs around(or runs away). Everythng seems ok, except he is not eating. And now he has diarrhea. Nothing changed in his diet, no loss of cage mate(he is alone), he is with us a year. Please, what is wrong?

Answer
I'm afraid I'm as confused as your vet. All signs seem to point to a dental issue, but if the vet examined the back teeth and said they were okay that seem to rule that out. I don't think the lack of eating caused diarrhea, I would be more suspicious that the antibiotics caused the diarrhea. Usually we give probiotics as well as the antibiotics to help prevent the intestinal problems caused by the medication.

This is just a guess, but ask the vet about doing an xray to see if he has a tooth that may be growing in an inverted area that might be preventing him from biting down on his food. I once had a sow who's teeth grew in a deformed way and actually began coming out of her lower jaw near her esophagus. The poor thing had to be put down because she couldn't eat, but I didn't know why. Once she was euthanized I was able to do a proper exam and found this bottom front tooth that had somehow diverted it's normal growth pattern and was preventing her from swallowing.

Of course this is just a suggestion because I can't even begin to think of what this might be. I wish I had a magic answer to this puzzle. Please let me know what the vet finds.