There's a bunch of reasons that women suffer PMS, and the time, and duration, that they suffer it (and even the length of their period) varies from woman to woman. The hormonal changes associated with menstruation basically can be a huge stress on the body - or barely be noticed at all.

PMS may be something that a woman has lived with her whole life, and never really thought that there was any way to alleviate or eliminate it. It's also something that someone women can feel extremely personal about, and can resent some bloke coming in to 'fix' it.

This page here talks a lot about it, but it's angle is pretty much from the clinical side, although it does talk about diet and exercise.

There's a whole lot of sites out there (Google 'PMS' - I mean, Google the word 'PMS', there is no site called 'Google PMS')

Here's a bunch of links I found helpful:

National Association for Premenstrual Syndrome (UK)
Women's Health Channel
womenshealth.gov
Groin protection

Your options are:

1. Stick your head in the sand, ignore it when it happens. Get a twisted thrill out of the monthly near-homicidal arguments
2. Spot the pattern - keep your head down and avoid interaction at this crucial time (hence this site)
3. Address the issue; come up with a exercise/diet/treatment solution that WORKS and makes your life and hers a lot better.