what are these?

I've never had stretch marks before but I noticed these on my boobs the other day, they're more prominent on one than the other. I've read that stretch marks usually start out as purple/red but these are almost white. Does anyone know what could've caused them? I haven't changed my diet drastically to make me gain any weight, which is what confuses me, and I've always had small boobs. They don't seem to have grown any, which is another reason why the apparent stretch marks confused me