Don't quote me on this, but apparently PMS is all psychological (not the physical pain, but the mood swings and bitchiness). Women have been conditioned to believe PMS exists and the way that our society deals and views menstruation causes the PMS ... ie. they expect mood swings, they get mood swings ... or any sort of mood swings are blamed on PMS.
They say that in societies that viewed menstruation as something worth celebrating and in societies that had generally a different view, PMS was not reported.