The hard lump in the left shoulder after breast cancer resection is considered to be caused by postoperative recurrence and metastasis, trauma, bone disease and so on. 1. Postoperative recurrence and metastasis: there is still the possibility of recurrence after surgery, especially if it is not early radical surgical excision, there is still part of breast tissue left, it is very likely to have recurrence and metastasis after surgery, for example, with lymphatic metastasis or bone metastasis, etc., which will lead to a hard lump in the left shoulder, most of them are painless hard lumps, and some of them may have pressure pain. 2. Trauma: swelling may be caused by trauma to the left shoulder, often accompanied by significant pain. 3. Skeletal diseases: for example, degeneration of shoulder joint bones, which may easily lead to osteoporosis and feel hard lumps in the shoulder. In addition, local soft tissue disorders, hematoma may also appear in the left shoulder with a hard lump performance, try to go to the hospital in time to cooperate with the doctor for examination and early treatment.