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.