Not sure how to even describe this but;
Have you ever seen or heard of a dog hurting their nail bed sort of inside the toe?
Bugsy has been really funny about one of his back feet when I do his nails and I couldn't see anything wrong. Then we have had a few incidents of a single yelp - once when he was kicking back after a good poop and the other day when he seemed to catch a nail on the carpet. Early last week I saw he was bleeding and I couldn't find the source but it seemed he was bleeding from the toe right at the nail bed, yet I could not see any cut other open wound anywhere.
Can he have broken it inside his toe or detached it on the inside? He is still a little wary when I do his nails but otherwise seems fine.