Breast pain is often the first symptom of pregnancy, occurring as early as one to two weeks after conception — technically, weeks three and four of pregnancy. That sore boob sensation peaks in the first trimester because your body is flooding with hormones. These hormones have an important job, preparing your body to grow a tiny human — a hungry little human.
To feed that hunger, hormones work quickly to prepare your breasts for breastfeeding. Blood flow to the area increases and your boobs grow larger. The cleavage may be pretty great — but this growth can also be painful, even causing skin irritation and itching. Ouch!
The milk ducts in your breasts also grow to prepare for breastfeeding. And hormones stimulate the growth of milk-producing glands. Basically, your boobs go through a massive growth spurt.(Healthline.com)