Endometri­um is the tis­sue that forms the lin­ing of the uterus and sheds dur­ing men­stru­a­tion. When endometri­um begins to form out­sides of the uterus, endometrio­sis occurs. Endometrio­sis can be painful as the blood from the shed­ding tis­sue has nowhere to go. Some symp­toms of endometrio­sis include cramp­ing, pain dur­ing inter­course, infer­til­i­ty and a heavy men­stru­al cycle.