It feels like I'm growing a tail, somebody please explain what's going on.?

[Octevio]

Fairly recently I find myself with a hard, painful bump around the small of my back where the tail bone meats the buttocks. The bump is barely visible to the naked eye save some red skin underneath the protruding area. It doesn't interferer too much but it is making it a tad difficult to sleep and I can feel it standing up. I wish I could be more descriptive. If anybody has a hint to what is going on I will be eternally grateful