Maybe you can have some physio while you are having your radiation treatment? Even a couple of sessions to show you what to do so you can then do it yourself. They should be able to advise you of a good physio where you are having your radio treatment as many people have physio while going to radiation.
I have not had reconstruction but find swimming great exercise for my arm and chest area (I had aux clearance and mastectomy in May 2013). I still do not have normal movement in this area (radiation really did me no favours in this regard) but it is improving with constant exercise and stretches. Hope you can get some assistance with this. Take care. Deanne xxx