Endometriosis is usually a long-lasting (chronic) disease. When you have endometriosis, the type of tissue that lines your uterus is also growing outside your uterus. The clumps of tissue (called implants) may have grown on your ovaries or fallopian tubes, the outer wall of the uterus, the intestines, or other organs in the belly. In rare cases they spread to areas beyond the belly.
With each menstrual cycle, the implants go through the same growing, breaking down, and bleeding that the uterine lining (endometrium) goes through. This is why endometriosis pain may start as mild discomfort a few days before the menstrual period and then usually is gone by the time the period ends. But if an implant grows in a sensitive area, it can cause constant pain or pain during certain activities, such as sex, exercise, or bowel movements.