It sounds like something that I saw once at the local reptile shop. They had a Woma Python in a tank with a very large bump on his back. I asked the guys about it and they said that the previous owner didn't pay attention and the guy got bit by a rat. They said that during shed that area doesn't shed well and he needs help with it.

Beyond that I know nothing about it, are you using anything on it? Like Neosporin or ReptiWound?