Premenstrual dysphoric disorder (PMDD) is a mood disorder characterized by emotional, cognitive, and physical symptoms which cause significant distress or impairment in menstruating women during the luteal phase of the menstrual cycle.

It's currently estimated that around 20% of menstruating women suffer some symptoms of the disorder. Women with all the symptoms may be as high as 5% - 8%.

There is currently no agreed theory as to the exact cause - but it's assumed to be due to sensitivity to fluctuating levels of certain hormones (i.e. the reproductive hormones), environmental stress, and genetic predisposition.

Mood symptoms include emotional lability (rapidly changing emotions, sensitivity to rejection, etc.), irritability and anger that may lead to conflict, anxiety, feeling on edge, hopelessness, difficulty concentrating, appetite changes, sleeping more or less than usual, or feeling out of control. The physical symptoms are similar to the symptoms of PMS. These include breast tenderness or swelling, joint pain, muscle pain, gaining weight, or feeling bloated."

Source : Wikipedia