It could be that your period is about to start; breasts can become more sensitive and tender around the time that a girl starts her period.
Or it could be a sign that you are growing. Some girls will get hurt in their nipples when they are around 13 to 15.
really it depends on what ur body reacts to it could be anything from stress to abnormal period timing
My breasts and nipples get a little bit sore for a day or so around "that time" of the month.
How do your breast grow like how would u know? Or when do they start growing?
some one has probably been sucking on them