To get the uterine lining out of your womb during your period, your uterus has to contract. Sometimes, when the contractions are stronger, your womb squeezes its blood vessels so tight that oxygen can't reach them, which sends pain signals to your brain (ouch). But wait, there's more – this process increases your production of chemicals called prostaglandins, which encourage more contractions of the uterus. There really is no mercy.