Myofascial pain syndrome.

Myofascial pain syndrome is a chronic pain disorder. In this condition, pressure on sensitive points in your muscles (trigger points) causes pain in the muscle and sometimes can cause pain spread to other  parts of your body. This is called referred pain. Trigger points can also cause stiffness and weakness.
This syndrome typically occurs after a muscle has been contracted repetitively. This can be caused by repetitive motions used in jobs or hobbies or by stress-related muscle tension.
While nearly everyone has experienced muscle tension pain, the discomfort associated with myofascial pain syndrome persists or worsens. Treatment options include massage therapy, dry needling, and physical therapy. relaxation techniques such as meditation also can help.

Lats and poor posture.

One of the often overlooked muscles affected by poor rounded shoulder posture is the latissimus dorsi muscle. They are the largest muscles in the back and function primary to adduct the arm and to assist in medially rotating the arm. Chronic strain causes trigger points to build up in the muscle tissue causing pain, referred pain, stiffness and weakness. Trigger points in the lats refer pain into the back, shoulder, and down the arm into the fingers.