There is a bone lump in the back of the head, which feels as smooth as a joint. You can't sleep on your back, or you will feel uncomfortable. What kind of bone is this?

According to your description, there are two possibilities: one is basically the occipital protuberance, which is the normal structure of the human body, but yours is relatively large and affects life, which can be ignored. Another situation is that skull tumor, namely osteoma, is a benign tumor of skull, which is easy to grow up and affect life. In either case, since it affects your life, I suggest you go to the hospital for a head CT to see the specific situation.