I have a pain under my left breast, it's painful to the touch and hurts when I flex my abdomen muscles and when I move certain ways. What is this?
It's not your breast
The rib cage and chest wall can become sore or strained, especially after vigorous exercise.
Did you just join a crossfit class or do 100 push ups? If you are able to take Advil (ibuprofen) or Motrin, usually this will help.