I haven't been to a doctor for a while now, but for the past month, I have noticed under my right breast, or at the edge of it, it is so sore. I have a little boy, so I am constantly picking him up, and that's when I notice. I am sure I'm getting ready to menstruate soon, but it's just on the bottom of one of them. The other breast is fine, and it's only sore underneath the right breast.