In normal females the lining of the uterus builds up in preparation for pregnancy. If pregnancy does not occur, the lining then sheds itself which results in the menstrual period. Endometriosis acts just like lining in the uterus for the first half of the cycle, building it's self up, but is not able to shed the same way. It may attempt to shed, causing localized bleeding and building up pressure, which may be one cause of the pain.
The two main symptoms of endometriosis are pelvic pain which may be similar to menstrual cramps and infertility.
