Why does pain from a headache spread to other parts of my body?

Chronic pain disorde. You could be having chronic pain disorder rather than a headache disorder. The mechanism of chronic headaches and of chronic pain involves several factors including sleep disorders, genetics, nutrition, behaviors, stress, exercise, and muscle contractions. You may require a combination of physical therapy, posture training and short term medications. Pcp might be a good starting point.
Sensitive Nerves. Many headache conditions such as recurrent tension type headaches and migraines come along with increased body pain sensitivity. When your head hurts all of your pain nerves are more sensitive and you body especially your neck and back can hurt as well.