system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do periods hurt?

Like the title says, I'm curious as to what exactly happens when periods come. Why do women get cramps during their periods? Why does it hurt so much, when it's just blood coming out from there (Which sounds bad enough on their own)?
gpt
A females uterus has a layer that’s made out of muscle. During the time that the uterus expels blood through the vagina, the uterus is contracting to help the blood move out of it and the muscle contractions are what hurt.