Thanks everyone for your help!
Yes, we need to get to the vet - was hoping to postpone it a bit until we finish reeling from Christmas' expense.
He doesn't seem to be bothered overmuch (no head shaking) and at first I thought that it was where he'd bonked his head off a drawer I was opening. But then we found the one under his chin, so we took a closer look.
Anyways, thanks again with your suggestions!