Answer & Explanation:
When you're on your period, you uterus is shedding it's protective lining that it formed previously in case the egg was fertilized. Because it no longer needs the lining, it sheds it. The lining is made of blood and other matter, which is why you "bleed" during your period. In order to shed this lining, your uterus must expand and contract, kind of "shaking" the lining off. This expansion and contraction cause period cramps and pains.