Endometriosis is a condition where cells like those in the lining of the uterus are found outside of the uterine cavity, typically on the ovaries (though they can occur in many other places). During the monthly cycle oestrogen stimulates the endometriosis to grow, then break down and bleed. This internal bleeding can't leave the body as a period and so leads to inflammation, pain and the formation of scar tissue.
Orthodox medical diagnosis is via laparoscopy, while Chinese medical enquiry may consider that an internal state of 'damp' or 'blood stasis' is causing an obstruction in the reproductive system. Possible causes include hormone imbalance, immune dysfunction, menstrual irregularities, genetic predisposition and emotional issues.