Muscle Strain: This is the most common cause of neck muscle tenderness, often resulting from overuse, poor posture, or direct injury to the muscle. The chronic nature (a month) suggests a possible ongoing strain or repetitive injury.

Cervical Spondylosis: This condition involves wear and tear on the discs in the neck, which can cause muscle tenderness due to the body's attempt to stabilize the spine.
Tension Headache: Often associated with neck muscle tenderness, tension headaches can cause pain that radiates to the back of the neck.
Fibromyalgia: A condition characterized by widespread muscle pain, fatigue, and tender points, which could include the back of the neck.
Poor Posture: Chronic poor posture can lead to strain on the neck muscles, resulting in tenderness.

Meningitis: Although less likely, meningitis is a serious condition that can present with neck stiffness and pain, making it crucial not to miss.
Cervical Spine Fracture: Especially if there was a traumatic event, a fracture could cause muscle tenderness and is critical to diagnose promptly.
Spinal Epidural Abscess: A rare but serious infection that can cause neck pain and stiffness, among other symptoms.
Cancer (Metastatic or Primary): Though rare, cancer involving the spine or soft tissues of the neck can cause persistent pain and tenderness.

Polymyalgia Rheumatica: An inflammatory condition that causes muscle pain and stiffness, typically in the neck, shoulders, and hips.
Rheumatoid Arthritis: Although more commonly associated with joint pain, rheumatoid arthritis can also cause neck pain and stiffness.
Ankylosing Spondylitis: A form of arthritis that primarily affects the spine, leading to chronic pain and stiffness.
Neck Muscle Hematoma: A collection of blood outside of blood cells, which could occur due to trauma or spontaneously in individuals with bleeding disorders.