It's totally normal. I'm 19 too and just this year I had jumped from a 34C to a 32DD! It just seemed to happen overnight, one night me and my boyf were just looking at them and we both thought wow, they're suddenly really big! lol
I don't think they stop for a while yet, maybe 20s or something. Breast size is heredity and also depends on body size, which can give a good indication to your eventual size.