While the age at which young ladies begin to develop breasts or get their periods seems to get younger, I can virtually assure you that at 13 years old, you have just begun the growth of your breasts, and over the next several years your body's hormone signals will be undergoing changes that will affect your breasts. This is why you notice fullness, or maybe even tenderness, in the breasts around the time of your period. The reason why a period happens is because the body creates different hormone signals at that time of the month, and some of those affect the breast tissue. Until those hormone signals stabilize for the long term, and this probably won't happen until you are around 17 or 18 at the youngest, the breasts may continue to change. Bear in mind too, that the breast is also comprised of fat tissue as well as glands, and that can increase or decrease depending upon a lot of factors too, like your genetics, your dietary intake and exercise, and hormonal signals as well. Some ladies actually report that their breasts continued to change into their early twenties. This is a large part of the reason why we don't like to do elective cosmetic breast surgery on girls younger than 18.