Unknown. The top theories in order: 1) implants form from back-flow of menstrual endometrium through the tubes into the pelvis, 2) disease spreads through lymphatics and/or blood vessels, 3) 'coelomic metaplasia, ' in which normal cells change into endometriosis. The first two likely account for many cases. The arguments get fairly involved, but none of these theories is entirely satisfactory.
Cells in wrong place. Endometriosis is a serious problem in which little pieces of cells and tissue (similar to the lining of the uterus that bleeds and sheds each month) are trapped inside one's pelvis and abdomen. With each menstrual period, the trapped tissue sheds and bleeds on the inside of the abdomen (the blood has no way to leave the body), causing much pain and cramping.
Two theories. Endometriosis is most strongly linked to ovarian cancer. One theory for this association is that there are common risk factors for endometriosis and ovarian cancer to co-occur. The other is that endometrial cells that have spread from the uterus to the ovary can transform into cancer cells via gene mutation. However, it is generally thought that endometriosis is not a direct precursor to cancer.
Tricky question. Experts don't agree if progression of endometriosis to cancer happens or not. It's tricky to design studies to answer this, and simply finding endometriosis with cancer in a patient emphatically do not prove one came from the other. If progression occurs it is an infrequent phenomenon. Many assume progression may be from estrogen exposures, but this is also just speculation.
Various. Reflux of uterine lining in susceptible women, fetal tissue that wakens up and develops into endometriosis are a two possible mechanisms.
Polysp are. The result of accelarated growth of the uterine lining (almost always benign) and not uncommon; endometriosis is different. Fragments of uterine lining go into abdominal cavity and implant on tubes, ovaries, abdominal wall, some women have altered immune system that (for whatever reason) isn't as adept at "removing" the tissue that is now in a foreign place; these implants are hormonally sensitive.
Feels like my pelvis and insides like my uterus and ovaries are on fire. What could be causing this? I have endometriosis, adenomyosis, and pcos.
Nerve pain. Classically it is taught that nerve pain is burning in nature so this sounds like something is causing irritation of the nerves to or in your pelvis. This could be associated with the endometriosis.
Pelvic Pain. You may have your answer right there. Endometriosis and adenomyosis (although you are young for that) can cause chronic pelvic pain. Acute pelvic pain can be caused by pelvic inflammatory disease or a ruptured ovarian cyst. Best to see your doctor for further evaluation.
Hormones or surgery. Hormones may control this pain, but you might need a surgery to eliminate as much disease as is seen or to remove adhesions.