Hi Caramelle,
I know every woman is different but normally for me, my periods last 6 days (heavy days then light),and I don't fall on the same day every month, why? the number of days change every month sometimes. Some have 28 days others 31. So I am confused as to why it falls on the 20th every month. It is best to count your days from the 1st day of your period to see when you will start. Most of the time it is between 28-30 days. Check it out the next time you are on your period. If you are between those days, then you are fine. Maybe the vitamins are setting you on that kind of cycle. And causing you to flush more out (to rid).
But that is Hada's guess. If your gut tells you it is something more then check out the circumstances.
Hada