I know that some women have arm swelling and pain after breast cancer surgery. why does it happen?

Lymph nodes: Doing breast cancer surgery the lymph nodes are removed. This causes the lymphatic fluid to back up in the arm.

Lymphedema: Arm swelling after breast cancer surgery used to be an all-too-common occurrence secondary to the removal of many of the lymph nodes under the armpit . The incidence of lymphedema increases in response to radiation therapy that encompasses this area.Modern techniques for lymph node evaluation (sentinel lymph node biopsy) remove far fewer lymph nodes, dramatically reducing the risk of lymphedema.

Lymphedema: If a person has had surgery in the axilla to check for lymph nodes, they can be at risk for lymphedema or arm swelling. This surgery disrupts the passage way for fluid to drain from the arm which then causes swelling. Thankfully, lymphedema from breast surgery is not a profound as it once was since less lymph nodes are removed.

Pain happens after: most surgical procedures. In mastectomy, a nerve that exits the side of the breast is usually injured or sacrificed and irritation of is causes pain or noxious tingling especially when the arms swing during walking. Swelling is independent of that and can be caused by removal of lymph nodes and exacerbated by not putting the limb thru frequent adequate range of motion- usually out of fear of pain