There are some possible reasons of why this occurs, but there is no one thing known to cause endometriosis. One explanation has to do with how the menstrual fluids move which causes small parts of the endometrial tissue to be in the wrong place. This can cause scarring and discomfort. When parts of the tissue are spread to other areas, there is an irritation which results in swelling and pain. What will happen if I don't get treatment You may continue to have pain, you may have difficulty getting pregnant.
Some people have no signs or symptoms at all.
Half of the women that do get treated, say that there pain is much improved. Women can discuss the need for treatment with their doctor and decide what or any treatment that is necessary. Shaw, R.W., Treatment of endometriosis.