I had the same and the lovely nurse said to use my arm as much as possible and do the exercises. I was getting to the point where I was frightened to reach out incase my arm went ping!
After a week, it does feel abit easier - I can pull my jumper up my arm without it hurting.
Good luck xxx
Mine appeared during rads which was 10 weeks after surgery. The Physio, linked to the radiotherapy dept, taught my Husband how to do it. I saw her for about 10 weeks and she massaged for about 45 mins without oil / cream going outwards in strokes from a central point like a star....It did hurt and it hurts a bit when my Hubby does it now but if he doesn't it comes back and is quite restricting.
Hope this is helpful. Maybe there is a Physio linked to you Radiotherapy Dept. Ours did the instruction on exercises during rads.... Take. Care Butterfly x