Stretch marks

Okay so recently I’ve noticed I have some purple stretch marks on my breasts. I know it’s normal for girls going through puperty to get them because they’re growing but I’m 17. I started getting my period when I was 11 and my boobs never really grew I’ve always been in an a cup. Is there any reason why I could be getting them other than maybe they finally decided to start growing? Which I haven’t really

Noticed any size change but who knows. I just want to know why they all the sudden appeared when I haven’t changed anything and my size isn’t changing either