I honestly think this old "time of the month" chestnut is exactly that.
Humans do not ovulate whilst they are menstruating, therefore, in spite of men alluding to it as hormonal, there are none of the necessary hormones being released, even were horse to be capable of making a HUGE cross species jump, mentally, to cause any reaction in a stallion.
I think it is far more likely that women, for whatever reasons, are emotionally fraught and short tempered at this time, and some stallions will use any excuse to misbehave or test the boundaries!!
