I found this a bit weird if not creepy. Turns out my SO, my regular ATF and my SB/FWB to boot, all have Auntie Flo visiting around the first week of every month.

Do your SO, ATF and SB all hang out?

So, while it's possible they could all sinc up once in a while, it can't be "the first week of every month."  Unless you are cursed.  Then again, you are clearly a pig so perhaps you are cursed.  So just find another girl to fuck.

So with that in mind, a word of advice. Don't ever try and outrun a bear, you can't. Just make sure you're faster than the other person.

Donald Trump may have some thoughts on the matter.

well if they all live together, then it could happen, but otherwise maybe just a weird coincidence or you did something wrong, they found out and now they think they are punishing you

And if she is a woman isn't she way past menopause

The very same could be said of Trump

But how about you show the universe what you're made of?

Sure, there is plenty of research on the subject for you to peruse!

For some reasons I thought I heard it happens when women having sex with the same man and not having to hang out together.

Yes technically 28 but of course it never works out at 28 100% of the time. Taken into consideration both plus and minus variation they all "average" out roughly the first week or so of the month.