That position for computer use is not ergonomic at all...

You probably strained your neck muscles with such a long time frame of use like that..

Have you seen a Dr yet? Most likely they should send you to Physical Therapy.. Most likely it is soft tissue/muscles, maybe some posture changes with forward head or shoulders added in..

Muscle spasms & Trigger points can cause pain.
Do you have any pre existing issues with your spine/disks?