there was a thread about a snake having blood on it after shedding, and from what i remember everyone said it happens, so like you said im guessing the skin got caught on the spurs (hooks on either side of the cloaca (hole)) and tugged them a bit, or the snake rubbed against something too hard and pulled it that way, but just keep it clean and keep an eye on the area to make sure there are no infections, you could even put neo-sporin (spelling?) on it, but im sure it'll be fine