How are the cat's teeth? Around that age, the teeth can have quite a build up of tartar and grunge and can abcess and become quite painful, and the cat may be hesitant to eat because it hurts. They can also swallow a lot of unpleasant crap from rotting teeth and this will make for an upset tummy. I assume the vet checked the teeth and gums? If the gums are overly red with swollen areas, this shows there is some degree of irritation or infection. Take a look at the teeth, even the back ones, and see if they are showing signs of brown/black (anything but white
) and foul smell, and this signifies some dental work is in order. This is just the first thing that I thought of after reading your post.
If the teeth are nice and pearly, perhaps another trip to the vet is in order and some blood work may be necessary to rule out other causes.
Another thought is: have you had any changes in your environment that might be upsetting the cat? Sometimes the smallest thing will throw a pet for a loop! Let us know what you discover.