Though it is very difficult to diagnose a lump without examination, I will try my best to list the possibilities and explain how each lump is diagnosed. In some people the occipital bone at the back of the skull has a protuberance. This is a normal finding. Another normal finding is a protruding spine of cervical vertebra due to bad posture. The other possibilities are a cyst, a neurofibroma (localized harmless swelling in a nerve), lipoma (localized collection of fat), a swollen lymph node, an abscess, or folliculitis (hair follicle infection). Rarely is it a bony tumor like osteoma. Since you have spasms, you could be having a pinched nerve.
Do discuss this with your doctor and get yourself examined. Take care!