What Causes A Small Thread Like Substance Under My Nipple?

I am a 45 year old female with 3 young children. I am still nursing my twin 3 yr olds (unusual, I know, but its only twice a day at most and it works for us). I recently noticed a rope-like structure under my right nipple. The orientation is from my chest wall to the center of my nipple. I can palpate about 2 cm of it. It is tender/painful when I nurse. It feels like an inflammed ductule, but I am concerned it could be something else. two weeks ago I had to take prednisone for 6 days for a poison ivy rash. The prednisone seemed to completely diminish this swelling, although I could still feel a smaller "thread-like" structure.

do you have any advice?