What Causes Core Pink Welt On The Right Side Of Breast?

I have a pink whelt about an inch long slightly raised on the right side of my breast right about where my armpit starts, but not in my armpit. I noticed it because the area is sore. I noticed it about four days ago. Although it never "itched", I originally thought it might be some sort of bug bite? I had a mamogram about three months ago, and all was well.