Why do I have cramps on my shoulder and neck muscle?

Heavy is the head... Your head is an immense object. Weights at least 15 to 20 pounds, as it's a large ball of bone filled with water and "other stuff". If your shoulders and neck are holding it in a bad posture, or in the same position for awhile, you'll strain the neck muscles. Typing long periods, driving, etc can do it. Getting a better posture, moving around, and changing activity will help.