What Causes Lump On The Coccyx?

I've had this thing near the top of my butt, it's not near the anal hole but rather on the top of my butt. Whenever I sit down, i have this feeling like a ball, or hard ping pong is at my tail bone... When I press against this ball-like thing on my butt, it feels hard. Around this small-pingpong-like thingy on the top of my butt, is skins... that are "swollen"? I don't know how to explain... I'm afraid to tell my parents.. is it cancer?... What do i do? I've had this thing ever since I was young... just never told my parents... probably 2-4 years now... help