I know the medical reason for this thanks to my recent BF class.
In our boobs we have alveoli kinda like in our lungs except they produce milk n colostrum... We don't have them until we get pg and the hormones activate their growth... Te alveoli go down into stores or reservoirs which lie behind the areola (the whole nipple not just the pointy bit)... The nipples Hurt because of the hormones and because of the stores stretching, growing and filling with colostrum/milk...