I only had a little breast removed so cannot relate to the weight loss issue on that side however I did go through a period of shoulder pain behind the scapula which was driving me barmy. My big son, about 180kg stood up behind my chair and gave me a massage in that area. He said he could feel like a knot there and while he was rough and it was painful he got right to the bottom of it and the pain went away.
After that I started to replace some of the tasks of my main arm which is the no breast side with my other arm. Just easy things like lifting things down from shelves and stuff. I think you do have to get some development in the slack arm so you get a balance.
Also in sleeping, I have noticed since there is no breast to impede my laying on that side, my arm tends to over curve in sleeping. I tilt my pelvis so I am not putting pressure on the shoulder while sleeping. I also got a pillow top cover for my mattress which has helped my shoulder and hips a lot.
I massage myself in the breast less arm and side many times a day as it just seems to like tightening up by itself. I still get phantom stabbing pains in my chest which are fleeting but that shoulder is good now.
