Women only please...?

Try squeezing your breast and see if any clear fluid comes out of the nipple. Also feel directly behind the nipple and see if you have a gland there, if you do then this is likely the cause. These are your mammary glands reaching final maturity. They are still heavily influenced by your surge in hormones which will not level off for some more years though the breast soreness/pain should subside soon. (within a year maximum). You can put warm compressed on it to ease the pain if it is that bad but generally you can just ignore it. Other reasons could be that you have pulled a muscle or you are sleeping awkwardly on your breast or even just wearing ill-fitting bras. The reason is likely puberty related--you think you are done growing but your hormones are not settled and just like when a woman gets pregnant and has a hormonal surge, the breasts continue to evolve and change and hurt. I am pregnant at present and my breasts hurt and I am LONG done with puberty and I have the same pain you describe only in both breasts. When I was your age I had the pain and I squeezed from where it was painful down (like milking a cow) and sure enough got to the nipple and had discharge.