What was your progesterone level? Before ovulation (usually tested on cd3), progesterone is normally low (less than 1.5 ng/mL) and in the luteal phase (usually cd21 or 7dpo) it should be above 15 ng/mL.
In a normal ovulatory cycle, your progesterone level goes up after ovulation, and if you haven't conceived, it peaks mid-way through your luteal phase and then starts to goes down and when it drops to a certain level you get your period. So with the provera (or similar progesterone type) drugs, you take it for 5-10 days and then when you stop taking it, you get your period a few days later. It's basically tricking your body into thinking that it's time to have a period, even if you haven't actually ovulated. If you search the forums for 'provera' or 'medroxyprogesterone' you can probably connect with other women who are taking or have taken this to trigger a period. Good luck to you
https://www.advancedfertility.com/progesterone-withdrawal-test.htm