That is a prolapse. Trying to deal with it on a gtp is going to be a chore.

Did it go to the bathroom recently? From what you described it sounds like it may have a large urinate or feces that is having trouble exiting the snake. Are you able to feel the area and see how hard the bump is?

One cause for such an issue is dehydration so make sure your humidity is where it needs to be.

I would contact the person you bought it from and make them aware of the issue and see what they suggest as dealing with unpassed waste is a delicate process and is going to be even more tricky in such a fragile snake.