Guinea pig tooth problems?

The past few nights we have been posting about our guinea pig's illness. Here are his signs/symptoms:

Eating less and slower

Irregular feces

Minimal eye and nose discharge

NO sneezing/labored breathing/drooling

lethargic

He dramatically improved after seeing the vet, but still had an appetite issue and had irregular feces due to having critical care (made it wet). His feces are back to normal and he is purring again, but still not wanting to come out of his house and is now making a weird noise with his mouth when he tries to chew, letting pellets and carrots fall out of his mouth. This frustrates him and he turns away (carrots are his favorite). The vet checked his teeth and said even his molars were okay. Could we have done damage to something in his mouth by syringe feeding him incorrectly? What could this be?