Hello Davina, that was a very wonderful question you asked, and a very important one too.
In fact, you might be surprised to learn that PMS/PMDD is not all that different from so-called normal depression. To understand exactly why is to study the human physiology to quite some extent, including why and how we react to internal and external stimuli on both a physical and spiritual level.
This all may sound a bit complicated but it really isn't. The truth is that in both depression and pms/pmdd (and a host of other diseases and disorders) the most prevalent cause is basic stress. And stress in itself creates a domino effect on many body systems, including our hormones. This is why there has been a modicum of success with SSRI treatment in women with PMDD (although I do not recommend this form of treatment long term at all!). It is very important to realize that there are all kinds of stress, both healthy and unhealthy, and it is the unhealthy stress which is the most insidious and the most ignored culprit here (and very often hard to pinpoint or recognize).
It is also important when considering stress as the primary cause to not just look at it at face value, as there are many levels of stress and many co-factors which eventually culminate in many different kinds of physical disorders of the body. PMS/PMDD is very seldom a stand-alone disorder.
I hope this helps to answer your question somewhat. I do encourage you to read my essay on this subject in full as I go to some lengths explaining the role of stress in PMS/PMDD.
http://geocities.com/pmddandpms.html
All the best,
Anthea