The reason, due to the mature follicle rupture after ovulation, estrogen levels dropped sharply obvious, can not maintain the growth of the endometrium, causing the endometrial surface layer of localized ulceration, detachment, which occurs breakthrough a small amount of bleeding, with the formation of the ovarian corpus luteum, secretion of sufficient estrogen, progesterone, so that the ulceration of the endometrial surface layer of the endometrium quickly repair and bleeding stops, or may be when the ovulatory period, the mature follicle Secretion of more estrogen, resulting in endometrial congestion caused by red blood cell leakage; may also be in the umbrella of the fallopian tube when ingesting eggs, the blood-containing follicular fluid through the fallopian tube reverse peristalsis sent to the uterine cavity and then by the cervix of the uterus through the vaginal outflow.

You have regularity, so there is no need to take progesterone tablets