I was just recently experiencing a really annoying headache at the back of my head, only on the left side. I couldn't figure out why I had this pain in such a specific area or how to get it to stop. I finally had a massage and the massage therapist asked me several questions, including about my work/desk environment. With her help, I figured out that I was resting my left arm on the arm rest on my chair, and whenever I leaned on the chair like this my neck/left shoulder muscles were getting all cramped up, subsequently causing the headache. Now I avoid my arm rests and feel much better.