Probably you have muscle strain
as a result of your work scheduele which is worst at the end of the day because of excessive muscle fatigue
. You could also be having cervical spondylosis
. If all the tests have come out normal, you have to take pain killers off and on, some local analgesics, infrared or SWD therapy and do your exercises. You may also take short breaks and do relaxation exercises in between your work sessions.