I've noticed over the past few weeks that my dog is whining (as if in pain) while feeding on dry food. I usually feed him dry food and he eats at a fast pace. At first it would happen occasionally, but now it happens almost every feeding.

Some things I've noticed while he's eating: his stomach starts to contract, (cave in), and it will coincide with his whining/yelping. He isn't constipated nor bloated.

This morning, it was painful enough that he stopped eating, and began to pace around anxiously. After pacing, he'll go back and continue to eat. While pacing, he'll also hang his head down, and smack his lips.

Any ideas?