Hey Raymonet,
Just starting rads now so I cannot speak from my own personal experience. I work in the field of rehab though and recommend you see a physical therapist with experience with radiation effects. All of the above you mentioned will help - exercising, stretching, etc. you just have to do it the right way, in the right order, at the right time - a physical therapist can get you started on the right path to regain muscle use and decrease pain. When something hurts we do all kinds of things that are biomechanically wrong to compensate - in the end you're left dealing with the first problem and then the secondary issues that arise from improper compensation of movement.
Hope it's possible for you to see a physical therapist and get that shoulder/neck moving well again!
Jennifer