Well, it’s very simple. The uterus is your first home.
That’s where you start life, as a cluster of cells, tightly gripping onto the walls of the uterus. This is where you find nourishment. The uterus protects you inside your mother, absorbing all the shocks, and jerks (like an airbag in a car).
When you are old enough and the time is right, it creates a series of contractions to eventually push you out, and safely deliver you into the hands of a doctor. So, the uterus has the function of accepting, and nurturing a pregnancy, when it happens.
And when this pregnancy doesn’t happen, the uterus has the function of creating havoc: blood, cramps, and pain.
Now, the reason why this happens is that your uterus wants to get pregnant. All the time. So much so, that it builds a lining on the inside, (like a skin but on the inside). This layer is called the endometrium. So it keeps building up the endometrium on the inside, to make it nice and soft and cosy for the baby when it comes in.
Now obviously, you are not getting pregnant every single month (can you?) which means your uterus gets angry. It gets angry and then it throws out everything it has built up inside. The nice and thick and fleshy endometrium goes straight out of your uterus, through the cervix, into the vagina, and out of your body. And that’s a period! And let me tell you, periods can really be very very painful.