My boxer has a huge ball shaped tail?

Hi I have an 8 year old boxer, and with in the last year or so, she has grown some sort of growth. She has this huge round lump that has consumed her tail. Her tail was docked as a pup, but literally what she has left is what I said. Not sure what it is, it is not filled with fluid or anything, as a matter of fact it is hard like a rock. It is also very heavy, due to this whenever she goes to the bathroom outside, we have to wipe the stuck poop.

If anybody knows what this is, or can shine some light on this... please help.