What causes pain in back of shoulder?

Many causes. Pain the the back of the shoulder can relate to neck arthritis, muscle strain or rotator cuff problems.
Various causes. There are various causes to it. It may be that there is something wrong with the shoulder itself (sprain/strain) or the pain may be a referred one (coming from the neck or the forearm). The worst case scenario is heart ache radiating to the shoulder. That requires immediate attention. To better sort this out, we will need more detailed information about your pain.

Related Questions

What causes pain in back between the shoulder blades and abdominal aches.

Stomach or pancreas. Pain in the back between the shoulder blades associated with abdominal pain can be an esophagus or stomach or pancreas problem. Please see you health provider for assessment.

What causes pain in the breast, back and shoulder? Is the most likely cause cancer if I know I didn't do anything to hurt it? Cancer?

No. The most likely cause of musculoskeletal pain is not cancer. I see cancer patients, so I am always thinking about cancer, but primary care physicians see patients with these symptoms all the time. If they resolve they are likely musculoskeletal problems (arthritis, etc) but could be viral, etc. If worsening or not resolving then more concerning. See physician for follow up.
May be muscles. Given your age and symptoms cancer is unlikely. You may have a strain in your neck and shoulder muscles, likely the trapezius, periscapular and sternocleidomastoid muscles. These can remain tight, painful & sore for many weeks after overuse. They can cause headaches too. You might want to have a primary care doc check you out. Ice, ibuprofen, gentle stretching & range of motion exercises can help.
Probably not. Pain is not a common symptom of breast cancer. However you should alert your friendly primary care physician to these symptoms that you are having.