Many girls seem to get 2-3 different stages of breast growth. A friend of mine more than doubled her size in her mid 20s for no discernable reason and she's massive now. I've known/seen multiple other girls who have experienced the same thing many years after going through puberty already. The model "shyslut0" had this happen in the last few years, for example. I know my friend was on birth control but only for a number of months years back and hasn't taken it in a long time.