Does it? [Sans can't really see his head wound, for obvious reasons.] I, uh, kinda banged my head turning the car. Should probably leave the driving to you. [So his head is definitely pounding, but he isn't sure if that's blood-related. Still, assuming Papyrus means his arm, he moves enough to roll back his sleeve. It looks like someone tried to eat his arm, because that is exactly what happened. Aside from the bite, there's long furrows where little fingernails clawed through skin.

If Sans had been in his normal body, he'd be dust right now. He's trying not to think about that.]

Maybe I should run it under water or somethin'. [Water cleans things, so it probably also cleans something like this?]