The pain also might be coming from the position they have your arm up in during radiation. I remember the unusual positioning of the arm on my cancer side though I had mastectomies and if you've had pain prior to radiation it's likely the unusual positioning of your arm for the duration of the treatment. I recommend very light stretching exercises moving your arm slowly up the wall "walking your fingers up the wall" and back down. It will help loosen the tissues that were affected by the lumpectomy. And I also agree it will lessen over time.
This may lessen in time .... afterall this is your first treatment. As you no doubt read in the discussion it isn't unusual to experience some pain although it isn't always present. you might ck. with your Oncologist for some measure of help. Time changes many of the effects of Radiation Therapy so you can never really know about the end result when your course of therapy is completed. Regards ....