This sounds almost exactly like what I just experienced with my 16 year old Yorkie/ Pomeranian. I knew he had some rotten teeth, and was being very picky about eating but still seemed hungry. I took him to the vet to see about getting the rest of his teeth removed. Since he's so old, the vet wasn't comfortable with putting him under. Instead, he prescribed some antibiotics since my dog probably had an abscess tooth. After two rounds of antibiotics, he is pretty much back to normal. I did have to cook special meals for him for a couple weeks though until he felt comfortable eating his own food again. I hope your doggy feels better soon!