What Could Yellow Balls On Breast Nipples Indicate?

Hi I'm a 16 year old female, 5foot 7 weighing 12 stone when I weighed myself at the doctors 2 months ago and have just found what looks like a little yellow ball located very tiny in the centre of my nipple. My boobs have been a bit tender lately but I just gathered it was because I am still growing. I have been having periods scince the age of 10 and am on the pill to help steady them. What could this little yellow ball be?