Cramped.

Some things in nature seem so unnecessary. Like menstrual cramps. As if it isn't enough that women give birth, we have to be reminded about it every 30 days. Knowing why it happens may not really make it worthwhile (hah!), it may even piss you off more, but if you're lucky it'll at least distract you for a few minutes.

Every uterus has a bloody lining
So us mammals have this thing called the uterus aka the womb and in lay terms that's where the baby hangs out for nine months. Unfortunately, even if you're not pregnant, things are still happening in there.
The protagonist in this violent story is a slimy fellow called the endometrium (ENDO-MET-RIUM). So the endometrium is the inner lining of the uterus which gets recycled every month since your first period - which is from when, as unnecessary as it sounds, a woman is ready to bear child. So every month, your body is (somewhat stupidly) prepping itself to get pregnant. And since pregnancy is signalled by the embryo (egg+sperm=zygote -> embryo) embedding itself on the uterus wall, the uterus wall needs to toughen up in anticipation of this happening at any moment. After about four weeks of waiting and no victorious sperm emerging even after ovulation (monthly release of eggs - best time to get pregnant, unless you're Monica and Chandler Bing), your uterus finally gets the hint and realizes it thickened the endometrium for nothing. And apparently, the endometrium has to be fresh to successfully embed the embryo so we can't just have it getting stale waiting for you to get pregnant. So now the body has to get rid of this tissue doesn't it. And what better way to do this than bleed through your privates! Hormone levels (in this case decreasing estrogen and progesterone) signal the endometrium to get swollen and begin shedding. That, in a nutshell, is what happens every month during your period.

'If suffer we must, let's suffer on the heights'
Pain, blood's loyal ally, has a good reason to be here too. As the old lining starts to weaken, chemicals called prostaglandins are released. Prostaglandins prod the old lining into actual disintegration. They do this by causing the uterus wall to contract (imagine flexed biceps) and thereby restrict blood flow and oxygen to the outdated endometrium. The endometrium, as a result wither away and die. To get it out of our system, the walls keep contracting so that the waste tissue ultimately gets squeezed out of the vagina. As you can imagine, this squeezing is nothing but the cramp that we love to hate, made further painful by clots of bloody tissue trying to escape the narrow cervical canal (that leads the uterus to the vagina).

That's all there is to it. Of course this is highly simplified and each step I've described is actually governed by dozens of hormones and regulatory mechanisms, some of which illustrator Wendy Bryan has described in a fun way here. A more detailed account can be got here.

Oh and there's this totally stoner menstruation video i found that's highly annoying but so damn funny I just have to share. It could be useful to visualize but i'm not entirely sure what that egg is doing there.
Oh, and I'm not a doctor so do let me know if i'm wrong or if any of you know anything I don't.