All I know is this has to do with the lymph nodes and blockage or damage to the lymph vessels. Swelling occurs because the lymph vessels are unable to carry the lymphatic fluid away from tissues. Radiation can make it worse. Please let your doc know of this asap as can lead to bigger problems. Or have you already?
Any of us who end up having surgery to remove lymph nodes have a higher chance of experiencing lymphedema, and going through radiation/chemo increases the chances.