Hard bony lump on the back of my head! is it cancer?

See Dermatologist: A hard bump in the head could in fact be one a several things such as: lipoma, benign cysts, foreign body granuloma, Various bony/soft tissue tumors, or even infectious processes. The best way to rule out a malignancy/cancer is to see a dermatologist. He/she will do an exam and determine whether or not a biopsy is recommended. Since you are very anxious, it is best to schedule an appointment.

Probably not: It is most likely a place where you took a bump and the little bruise calcified / ossified. These are very common. You may have overlooked the injury. Show your doctor next routine visit.