Left arm sore an side around/under breast if i lift breast up its tender when i straighten my back/relax shoulder it hurts what could be causing this?

Musculoskeletal: It sounds like a musculoskeletal complaint. I would recommend seeing a chiropractor or osteopathic physician. You can also try stretching. If pain gets worse or associated with dizziness, shortness of breath, or other symptoms then consider follow up in ER

Arm Pain: This certainly sounds like it is likely to be musculoskeletal pain but certainly warrants evaluation by your physician.