Could you be pregnant? It is possible to still have your period and be pregnant. You might want to get tested if you have been having sex. That will cause your breasts to grow and become tender.
How old are you? Women do not usually stop growing completely until their late 30s or so. It really depends on how your hormone levels change though. Women who were flat chested in their teens, may start to develop more breast tissue when they are 30. Everyone is different.
Changes in diet can also have an effect on your breast tissue. Breasts are, after all, predominantly fat. If you start to gain weight, so will your breasts.
Some times birth control makers will make changes in their birth control. If this happened, that could also be the reason for sudden changes in your breasts.