Adenomyosis is essentially thickening of the lining of the uterus, the endometrium. The endometrium thickens and grows into the muscle of the uterus.1 During the menstrual cycle the normal lining and the overgrowth causes excessive bleeding. Furthermore, the bleeding of the abnormal growth occurs directly into the muscle, which causes pain. This blood accumulated in the muscle causing swelling, irritation and cyst formation over time. This can affect your entire uterus or it can be localized to one area.