Why can arm swelling and pain happen after breast cancer surgery?

Lymphedema. An essential part of breast cancer staging is the removal of some of the lymph nodes under the armpit. The lymphatic system's job includes draining tissue fluid and bringing it back into the circulation. Lymph node removal short-circuits this process leading to the risk of chronic swelling, or lymphedema. The less nodes removed, the lower the risk.
Lymphedema. When lymph nodes are removed from under the arm during breast cancer surgery, you are at risk for developing lymphedema (swelling). The amount of lymphedema varies and can depend on the number of lymph nodes removed. Lymphedema can be treated and controlled. I recommend you ask your surgeon to refer you to a lymphedema clinic for evaluation.
Lymph node drainage . Usually breast cancer surgery associate with lymph harvesting. Depending on how extensive the lymph node dissection is, you may experience pain and swelling of the arm and shoulder with extensive lymph node dissection.