The introduction section in this paper https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4727734/ says that there are specific ways to identify if it's a cervicogenic headache. These are things that your doctor should examine and confirm. They are trained to do that. I can't comment on it. When my neck and shoulder pained, it was brief, and not correlated with the eye strain. It was because I used the mouse and keyboard for too long. Taking rest and ensuring a reasonable amount of meat in my diet would make the neck and shoulder pain disappear quickly. It was never as prolonged as the eye strain. I eventually had to quit my job. In my case, needing to focus on small areas of the screen was what tensed the eye muscles. Watching a video or movie for longer duration caused much lesser strain. My left eye would often pain more than the right eye. Sometimes it'd be the opposite. I'd suggest asking your doctor for advice (or consulting a different doctor) and also seeing if my suggestions of getting proper uninterrupted sleep, periodic rest and proper nutrition helps in any way. If your strain is severe, it'll take a long time for you to recover. Don't expect immediate solutions. There's no point discussing this on Medium's comment section.