Suddenly sensitive scratch reflex after dog attack?

My dog (18 lbs, double coated fluffy mutt) was unfortunately attacked by my friend's much larger lab/pittie mix a few weeks ago, though thankfully he didn't sustain any punctures or injuries to his organs. He did, however, get quite bruised up, and it took about a week of pain meds and rest for him to not be sore.

Since then, I've noticed that when I pet him in the area where he had the biggest bruise, it sets off his scratch reflex. And I mean consistently. Always the same area.

I took him to the vet for a follow-up a week after the attack and they inspected him and found no issues with his skin. He's not acting any different either, just seems more sensitive on that part of his body. I asked my vet about this and he was stumped too, but didn't think it was concerning unless there were other skin or behavior changes.

Any idea why this would be happening? It's been about 3 weeks since the attack.