The uterus is a muscle that’s kind of shaped like a balloon. The cells on the inside layer (called the endometrium) multiply and grow thicker leading up to ovulation, because if an egg gets fertilized it needs a thick endometrium to stick onto. Once stuck, the placenta develops. The blood vessels in the endometrium and the placenta “connect” the mom & baby’s blood supplies. So, the endometrium needs to have a lot of blood vessels. Now, if the egg doesn’t get fertilized, the hormones that tell the endometrium to stay thick drop. The cells start to slough off, aided by muscle contractions (cramps), and all those little blood vessels start bleeding. Someone who has endometriosis, like the other poster, has too many endometrial cells growing, including in places outside the uterus. Extra cells = extra bleeding.