What happens when the egg after ovulation is not fertilised during a menstrual cycle?

During a menstrual cycle, the uterus starts preparing itself for a probable pregnancy. This is followed by the thickening of the uterine lining. 

One of the follicles matures and releases an egg during the ovulatory phase. However, if the egg does not encounter a sperm and remains unfertilised, it disintegrates. This causes the uterus lining also to degenerate. 

The shedding of the uterine lining causes bleeding which flows out through the vagina as menstrual blood.