I have a bump roughly the size of a grape near the top of my butt. It's hard, doesn't move, is underneath the skin, and doesn't hurt when you touch it or the area around it. It causes me no pain to sit or lie down but on occasion my lower back is sore in gerneral but that can be caused by something else. It might be a cyst, two family members of mine have experienced them, but it doesn't seem saclike to me.