Repetitive motion stress, pinched nerves, poor workspace ergonomics and a number of other physiological issues could be contributing. Sometimes massage therapy and stretching exercises are all you need (oh no, massage therapy, what a burden! ) but sometimes there are things like bursitis causing nerve pinching. I'd suggest seeing a physician.

If it only bothers you at work, that suggests ergonomics, repetitive stress, or plain old tension. Those are also the easiest to deal with at the least cost with the best results. If ergonomics is the issue, your employer may be required to address it (OSHA) with workspace adaptations - some people need standing desks, rubber balls instead of desk chairs, lower keyboard trays, etc.