When you start your period, your endometrium begins to slough off since no egg has been fertilized. At the same time that your endometrium is shedding, prostaglandins are released. These prostaglandins play a role in the contraction of the uterus in order to adequately shed the endometrium for the current cycle. With these contractions, people can have pain, especially if someone's uterus contracts frequently. There's also thoughts that certain stretch receptors can get activated when this happens, and that can also contribute to pain.
TLDR; a chemical is released when you start your period that causes your uterus to contract and that hurts
starry_flower t1_ixd6tzw wrote
Reply to comment by Deedle-eedle in When a person gets stomach flu (viral gastroenteritis) or food poisoning, what exactly is happening in the body that causes stomach cramps? by geekonthemoon
When you start your period, your endometrium begins to slough off since no egg has been fertilized. At the same time that your endometrium is shedding, prostaglandins are released. These prostaglandins play a role in the contraction of the uterus in order to adequately shed the endometrium for the current cycle. With these contractions, people can have pain, especially if someone's uterus contracts frequently. There's also thoughts that certain stretch receptors can get activated when this happens, and that can also contribute to pain.
TLDR; a chemical is released when you start your period that causes your uterus to contract and that hurts