Is in in your arm pit? I found a lump in my arm pit a little over a year ago. I had a mammogram and saw a breast cancer surgeon. He biopsied it and it came back as a benign fibroadenoma. I have it monitored about every six months, however, I was scheduled to have it checked and possibly removed last month, but I was pregnant and cancelled the appointment. Unfortunately, I had a miscarriage. But, it was really sore when I was pregnant. The hormones can make these tumors grow and be sore. Now, that my hormones are dropping and I have lost all of my pregnancy symptoms, it isn't as sore.
Ur milk ducts can extend under your arms. Mention it to ur dr and they will monitor it. I had this during pregnancy number 3. It turned out to be nothing and went away after birth.
I agree with the others. It is probably nothing to be concerned about, but any lump at any time should be reported to your Dr. All the best to you!