One boob bigger than the other help!?

ok im 13.. im only a A cup but one is bigger than the other and it is noticeable but not by that much like iff i flex the smaller side its the same size as the other. i don't know why it is like that the left on is bigger and that is the size i sleep on.. i don't no if that is why or something else but can you guys help me out. like help me even them out... please help its really weird