Whenever a woman is in a bad mood, is upset, is argumentative, is emotional in any way, inevitably someone will make a crack about how she must be menstruating or PMSing or whatever.
This is really frustrating and incredibly sexist. If I am rightfully angry about something this is used to completely invalidate my feelings and to shut me up.
So it is even more frustrating when a fellow woman says this to me. Stop perpetuating this nonsense! When I am angry, it is usually because someone has angered me, not because of my monthly cycle!
So try not to belittle the women in your life by telling them their feelings are just irrational hormones, because it is quite offensive.