Thank you for your question. Although without being able to examine you I can not offer you the specific advice on diagnosis and treatment that you need, but I would try to provide you some relevant information about your health concern.
Arm pain and neck stiffness may appear due to nerve irritation in the cervical (neck) spine because the nerves that supply the arm also start in the upper cervical spine. Mostly, this kind of pain aggravates after turning head and typically occurs in patients with herniation of cervical disc and in patients with marked arthritis of the upper cervical spine. In addition, this pain exacerbates with prolonged sitting in a stooped posture and when working on computers for long hours so this activity should be avoided. Since you are presently suffering from severe pain, please arrange an appointment with a neurologist right away for further assessment & treatment. Your doctor may suggest wearing a cervical collar or suggest some neck exercises as well. Hope this helps.
Take Care & Regards!!!