What Causes Stooping Of Shoulders After Undergoing Double Mastectomy?

Hi and thank you. I had a double mastectomy about five years ago. My tests are good; however, my shoulders have begun stooping. I JUST HATE IT. Is there a brace I could wear to remind me to hold my shoulders up? I am 80 years old, but please don t hold that against me! I attend a wellness center and take a water class almost three times a week and have been doing that YYYY@YYYY for over a year.