Hello, I Have What Looks Like A Swollen Tendon Or

hello, I have what looks like a swollen tendon or muscle (long and squishy tissue) stretching from my outer armpit area to the side of my breast. It is extremely tender to the touch. I have been to two doctors now and they claim it is excessive breast tissue but what I've read online is that axilary breast tissue is more like a ball or kiwi shaped. Any insight on what this is and why it hurts?