Breast cancer may cause arm pain. Breast cancer may cause arm pain because of axillary lymph node metastasis, bone metastasis or axillary lymph node dissection during surgical treatment. 1. 3. Axillary lymph node dissection: If axillary lymph node dissection is performed during surgery or radiation therapy is performed after surgery to irradiate the lymph node area, it may lead to obstruction of lymphatic reflux on the same side and lymph node edema in the entire upper limb, which may also lead to symptoms of arm pain. In addition, arm pain may be caused by the patient lifting heavy objects with the arm, or by the patient sleeping in a wrong posture, or it may be caused by a collision.