What is the reason of pain behind neck?

Neck pain: This can be due to spasm of the neck muscles. Sleeping the wrong way is a common cause. Straining of the neck muscles can also cause similar problems. Disc herniations usually have pain radiating down the arm. Would see your pcp if this persists.

Muscle spasm?: There are a number of reasons for that type of pain. Cervical spine disease , muscle spasm to prevent movement of a injured or inflamed neck are amongst the commonest causes. Viral infection "wry neck " is an uncommon cause too.