Every 28 days some blood and other products of the disintegration of the inner linning of the uterus (endometrium) are discharged from the uterus " menstruation"
During this time a new follicle begins to develop in ovary.
After menstruation ceases, the follicle continue to develop secreating estrogen which causes...
   1-The upregulation of progesterone receptors .
    2-The endometrium to become thicker.
    3-Rising the level of LH causes developing of the egg within the follicle.
    4-After 2 weeks there is a rise  increasment in the production of LH to trigger ovulation ( the release of the ova from the follicle)....under the continued influence of the LH , follicular phase occur , where the follicles are ruptured ( after reaching a certain size ) producing the ova and the corpus leuteum .
This corpus leuteum secreat progesterone which inhibit the development of a new follicle , inhibit the uterus contractions and continue the preparation of the uterus to pregnency.

Note that ...
No corpus leuteum,no progesterone.
No estrogen,no progesterone.

If fertilization doesn't occur..
The rising level of progesterone inhibits the release of GnRH which in turn inhibits the production of progesterone, so , progesterone level drops , the corpus leuteum degenerate , the endometrium begin to break down (programmed cell death"apoptosis"), the inhibition of uterus contraction is lifted and finally bleeding starts .