Any ideas why the arm swell, or become edematous, after a radial mastectomy?

Lymphatic drainage. After a radical mastectomy many of the lymph nodes have been removed in the armpit or axilla. These same lymph nodes that drain the breast can also be involved with draining the arm. After removal of these lymph channels the fluid in the arm must find other paths(veins or other lymph channels) to return to the heart. This can lead to swelling. Consult with your surgeon. They can help.