In endometriosis, the tissue that lines the inside of the uterus (Endometrium) is found outside the uterus, most commonly on the fallopian tubes, ovaries, outer surfaces of the uterus or intestines and sometimes on the surface lining of the pelvic cavity. Rarely, endometrial tissue may spread beyond the pelvic region like surgical scars, lungs, breast and other remote sites.
Anatomy of Female Reproductive System

In the first half of the menstrual cycle, the lining of the uterus (Endometrium) thickens under the influence of hormones (Estrogen and Progesterone) to prepare for possible pregnancy. If a pregnancy doesn't occur, there is a decrease in the levels of these hormones and the thickened lining of the uterus is shed out as menstrual bleeding.

The levels of the hormones also influence the endometrial tissue that is located outside the uterus. Depending on the levels of these hormones, this tissue thickens, breaks down and bleeds each month. Some of the blood can become trapped in the tissues and may not be able to escape. The blood breaks down and their end product leads to tissue irritation causing pain during your period. The trapped blood can also lead to the formation of cysts, which in turn may form scar tissue and adhesions that can cause fertility problems.