I noticed similar pain days post my surgery (only had lumpectomy and sentinal node removed) and it was getting worse as days progressed.
I realised it was connected to my using that kidney shaped pillow breast care nurse gave me to help protect the wounds from arm pressing on them. What Inworked out is that the material which is very satin like was making the little strap it was holding onto slio off my shouldre all the time and I was holding my arm and shoulder in a very tense unnatural position.to stip it from slipping off the shoulder.
So I stopped using it or at least when I did use it I did not use thebshoulder strap and instead just had it under the arm and my oressed arm held it in place. Once I started that - ie reduced use and changed how I used it pain was soon gone...
Not sure if U R using that small pillow or experiencing same issue but thiught I would share this just in case...
Jel.