sure it's normal, I was an A cup when I got my first period. I'm nearly a c now and it's been about 2 1/2 years since I got mine. supposedly they grow a little every 3 months, i'm not sure if I completely believe that but thats what some doctors say. Also that they're gonna grow until ur like twenty so you probably have some time
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Good Luck!
Pm me if you'd like, and don't put too much thought in it just be happy the way you are!