It is not exactly known what causes PMS. Common sense indicates it must somehow be linked to the fluctuating levels of female hormones experienced after ovulation. But the subtleties of why some women are more affected than others are not understood.
Normal fluctuations in hormone levels are responsible for some of the symptoms most commonly associated with the monthly cycle, such as bloating, breast tenderness or headaches.
Women who suffer from PMS may possibly have a lower than normal level of a certain chemical in their brain (serotonin), which may explain some of the non-physical symptoms such as irritability, depression and mood swings.

PMS is not caused by any underlying abnormality with the pelvic organs.

When should I seek treatment?

Recognising your symptoms are due to PMS is an important first step.
For the majority of women, the symptoms are a minor inconvenience you can recognise, anticipate and deal with yourself.

You may seek reassurance from your doctor, but do not necessarily need or want treatment. The value of such a discussion can be high and result in significant improvement in your symptoms.
For a minority of women, PMS is serious enough to affect work, daily life and relationships. If this sounds like you, you should see your GP to discuss your problems, possibly with a view to some treatment.
Women with severe symptoms who have not responded to simple treatments might wish to see a specialist. This usually means a gynaecologist, but a psychiatrist with a particular interest in treating severe PMS can sometimes be more appropriate.

What treatments are available?

There are many treatments for PMS, most of which have some short-term benefit. However, few provide relief for longer than a few months.
Treatment will depend upon the nature of the symptoms and their severity.
For many women, simple changes to diet and lifestyle, reducing alcohol and caffeine intake and cutting down on cigarettes will make the monthly symptoms more bearable. Your GP can give you guidance in this.
A suitable diet sheet is available via the National Association for Premenstrual Syndrome (NAPS) at www.pms.org.uk.

source: http://www.netdoctor.co.uk/diseases/facts/pms.htm