Endometriosis is a condition that affects a woman’s reproductive organs. It occurs when cells similar to those that line the uterus are found in other parts of the body. When a woman with endometriosis has her period, she has bleeding not only from the cells and tissue inside the uterus, but can also have bleeding from the cells and tissue outside the uterus. When blood touches these other organs, especially inside the abdomen, it can cause inflammation and irritation, creating pain. Sometimes, scar tissue can also develop from the endometriosis which can also contribute to the pain.