Good news! Your breasts continue to grow after you start having periods. Breast size is in part determined by hormones during puberty (which continue for about two years after you start your period) and by your weight. Most girls aren’t at their “adult” weight until a few years after their first period, and as a result breast development will continue. Pregnancy and breast feeding later in life also affect breast size, but those aren’t on the short-term horizon for you I hope!
I’ve heard that as well, but don’t know if it’s true or not. My breasts continued to grow until I was in college, but I don’t know if I’m typical. I’m curious what Dr. Molly has to say about this. Though, even if there are “rules” about when your breasts stop growing, there are always people who fall outside the rules.