Hello, Pain in the back of head is most often caused due to problems in vision. Also dehydration can cause headache. So try to keep yourself well hydrated. If this is solving the problem you need to follow the advice of keeping yourself hydrated. Speaking of vision, if you are using spectacles it might be possible that your error might have decreased or increased and its causing strain to your eyes. If you dont have spectacles then you might have got some amount of refractive error which is increasing when you do work causing strain to the eyes and hence headache. I would advise you to go an opthalmoligist to get your eyesight checked and follow up withe the same Thank you