S.B.
Your breasts might be itchy because they are changing not just hormonally, but perhaps also in size. Skin can itch as it gets stretched out and itch as it goes down again. Think of your tummy stretching with pregnancy and going back down after the baby is born. I think our breasts go through the same thing.
Try keeping a good moisturizing lotion on your skin and see a doctor if it continues. Especially if redness or rash develops.
Best wishes.