Lol it's more common then you think. It's from when they grew and your body just didn't have time to catch up.
Normal. You could try bio-oil if they bother you.
O.o how did that happen
O.0 it happens sometimes during excessive growth... Pregnant women get it in their bellies and fat people sometimes get them on their thighs and stuff
It means you lost weight from your boobs
when they grew