Whose blood is this?

This is the new game I like to play. By 'like to,' I mean 'have to.' And by 'game,' I mean 'awful moment of panic.'

A couple of times over the last month or two, while playing with the boy, I've spotted a touch of blood on my clothes, on a toy, somewhere. Then the race is on. Is it his blood? Is it mine? Has the cat finally lashed out, seeking her ultimate vengeance upon the cruel world?

Last night, he was climbing onto the futon, off the futon, dive-tackling the magazine I was reading, the usual stuff. Then I spotted a speck of blood on my t-shirt. Then on my leg. Then on the toy phone he playing with.

With two adults, this isn't so much a game as a simple, polite conversation:
"Good sir, are you bleeding?"
"Why no, it must be you."
"Oh, what a bother, it is me."
"Whatever might have happened?"
"I seem to have been lanced by a marlin!"
"Bloody hell!"

The boy doesn't take as kindly to such inquiries. One time, when I discovered he actually was the one bleeding, it was his hand. He's learned, through countless attempts at cutting his fingernails, that anytime I want to examine his fingers or hand it's something terrible and he should squirm and plead and running flailing into the night.

After several attempts, I managed to examine both hands, his face, the whole package, and he seemed more or less clean. That's only reassuring to the extent that it's not the child bleeding. There's still blood, and the cat was mysteriously furtive.

But I couldn't place it. Phantom blood - why do you vex me?