This is an everyday thing
It's your skin stretching! Don't forget to moisturize! :-)
I would say your breasts because they're growing.
Do NOT scratch it!!!! Lol no matter what or how itchy it is! Youll get really bad stretch marks. I have this one stretch mark on my belly that i had scratchbon occasion cuz it was that bad and now its my biggest stretch mark!!!!
It's just your breast tissue growing, the itch can be extremely annoying and at times unbearable but all absolutely normal