Premenstrually the lining thickens and is later shed if there is no fertilized egg to implant into the lining. A hormonal imbalance, infection or endometriosis can all cause a thickening of the lining, irregularly during your cycle.
Endometriosis is quite common in young women. your body is preparing to get pregnant every month that is why the lining thickens. menstruation is expelling the lining when you don't get pregnant....