1. Endometriosis
Endometriosis, a disorder in which uterine lining tissue grows in places other than the uterus, might cause your period to occur twice a month (besides the inside of the uterus). This tissue can disrupt periods, induce pain, and produce excessive bleeding

2. Pregnancy.
Some pregnant women suffer irregular bleeding, which may be confused with your period.
If you are sexually active, have two monthly periods, and suspect you are pregnant, the best thing you can do is obtain a pregnancy test, especially if you experience lower abdomen pain that seems to be menstrual cramps.

Read more>>>https://medicalcaremedia.com/5-medical-reasons-why-some-women-have-two-or-more-menstrual-cycles-every-month/