Lymphedema is the biggest pain in the neck. I've had to have therapy 2x's since DX. I know you have a little one, but try hard to favor that arm. Repetitive action can aggravate it as well.
Be sure your physical therapist is trained in lymphedema massage. She/he will able to teach you to wrap that arm and will also massage it properly.
There are also some good sites on lymphedema on the web, so maybe you can check those as well. With this disease knowledge is power, so the more you know....
Hope this helps, Luci
In the depth of winter, I finally learned that within me lay an invincible summer. Albert Camus