Endometrium is the lining layer of the uterus which sloughs off with each menstruation.
Bleeding into the abdomen irritates the lining membrane, the peritoneum, and causes abdominal pain. Bacteria can make their way up the vagina, through the uterus, and traverse the uterine tubes which open into the abdominal cavity. Inflammation of the lining of the abdominal cavity, the peritoneum, causes abdominal pain. Although there are many potential causes of PID, gonorrheal infection is one of them. Chronic Inflammation of the uterine tubes can occlude them resulting in infertility.