Have you tried Manual Lymphedema Treatment (also known as Lymphatic Drainage Massage)? I have not experienced any pain with this treatment, in fact, it feels good! They start out very lightly and progress according to YOUR feedback. My oncologist referred me to a therapist at the onset of my lymphedema. As previously mentioned, catching it early is VERY important.
Therer are exercises you can do, for which a therapist can provide instructions.