why are my boobs getting bigger ?

i’m 18. always been small chested, b/c cup since i was 13 and so i kinda thought it would always be that way. recently (past month or 2 )

my bf said they look bigger all the time now, i’m not gaining weight, not on birth control and 99% sure i’m not pregnant (condoms) and i start my period in 12 days. BUT is there any way like they are just starting to grow now ?