The most common cause is a pulled muscle – also known as a muscle strain. The muscle has been pulled or stretched past its normal limits, causing pain. Stiffness may also accompany this type of injury. The levator scapulae is most frequently pulled muscle in the neck.
Other causes of neck muscle pain may be due to whiplash. Whiplash is an injury, often associated with car accidents. The neck muscles are pulled past their normal limits quite quickly. Neck muscle pain may immediately appear or may surface days later. A direct blow to the neck may also cause neck muscle pain, such as bruising or damage to the muscle.
The neck muscles may also become irritated due to bad posture. Many sit at a desk all day, protruding their head forward. This is known as forward head posture. For every inch the head protrudes forward, an additional 10 pounds of stress is placed on the neck muscles. This can cause pain and other postural deficits.
You may also have a kinked neck from sleeping the wrong way. Read more about kinked necks here.