[WVU alpha phi]

My old dog did something like this once. My family went to the West Coast for two weeks and put him in a kennel (worst decision ever for him). When we came back, he had lost several pounds and the kennel informed us he had barely ate anything the entire time. We thought things would be fine once we brought him home, but he still didn't eat very much. We took him to the vet, turns out he had developed a stomach ulcer, the vet said probably from worrying that we had left him there for good. I was a lot younger, probably around 8, when this happened, so I don't know how the probably was fixed (I'm just assuming medicine) but it was solved and Pepper lived to be 16 years old.

However, since you went to the vet and they said everything was fine, I'm not sure. Maybe you should try to get a second vet's opinion? Or, since your dog will still eat certain foods, maybe it's just going through a phase?