Just my thoughts for what they might be worth to take or leave, no judgment or anything - just FYI stuff for future reference, etc.
Just because a dog, just like humans in pain, will eat, doesn't mean it's not hurting. We eat when we're in pain and so will many dogs unless they are in severe pain. They have pain receptors and feel pain just like any mammal but because they are, from the beginning of the wolf-to-dog species, genetically programmed not to show pain for fear of being attacked/killed for showing weakness, don't assume they don't feel it. If there is an abscessed, raw, infected, swollen, bruised or cut area on a doggie, it hurts just like on a human but they just cope differently. They just are programmed genetically to try to stay safe and calm by not revealing they are hurt, sick, sore, etc. A dog will a lot of times react more to something trivial that they don't like such as sudden hair-pulling during brushing or lifting him up unexpectedly or against his will by crying out than he will after actual open, cutting surgery. I've seen a dog with a traumatic amputation of a foreleg with a bloody stump wagging its tail and greeting people just as normally as you please. And he ate soon after awakening from surgery.
If a dog didn't feel pain, he would engage in dangerous behavior that led to lots of injury and eventually death, so like any animal that must fend for itself to live, they hurt, even when it's just a sore, swollen, infected tooth and gum.
Here's some more information - kind of a range of opinions, etc.
https://www.google.com/#q=do+dogs+feel+pain