I also had a double mastectomy with no reconstruction. I was given a whole set of exercises to do post surgery, but did develop lymphedema on the left side where most extensive surgery was done. I only had three nodes removed. The more you have removed, the stronger chance of developing lymphedema. I had a trained massage therapist work on it and it went away about 90%.
I also have a sore bump under that same arm, which my oncologist says is where they tie off vessels. The soreness comes and goes. I also had a seroma, which was painful fluid in the arm pit that had to be surgically drained. If you have lymphedema, best to get treatment right away.