I'm having heavy uterus lining discharge. what could be the problem?

Unopposed estrogen: Estrogens build up the inner lining, progesterone from ovulation then transforms the inner lining into a cushion for a fertilized egg. When there is no ovulation and hence no progesterone, the inner lining keeps growing. Eventually, the uterus starts shedding some of the inner lining, builds new/more lining up in other places, and the vicious cycle continues.