Breast pain during strength training?

Greetings all...I am a small cup female-32A-who has been strength training my upper body for about 4 months now, in an effort to better my chances as a boxer. After an evaluation by my personal physician, I started training wearing a fitted sports bra. The night after, and several days following a normal exercise routine, I had such an agony in both breasts that I could barely function or sleep. I had a mammogram, and tissue biopsy, neither of which showed anything. After attempting several more styles of sports bras, I began working out without a bra altogether, and the pain ceased to present. Has anyone else experienced this sort of issue?