Is My Sunken Armpit And Pinching Nerve Sensation A Concern, Given My History Of Breast Cancer?

I felt a pinchy nerve thing zip the muscle at the back of my right arm pit, then looked in the mirror and my armpit looks sunken. I am 72 and had breast cancer 25 years ago on the right side and had my lymph nodes cut in that area. I had little to no feeling there. Is this something to be immediately concerned about?