Below are some of the most common theories about why we experience PMS - keep in mind that more than one theory may apply to any individual.

Hormonal Excesses or Imbalances
The most popular theory regarding PMS is that the ratio of estrogen to progesterone is not in its proper balance and therefore can affect the target tissues. This may be caused either by the overproduction of estrogen or by a deficiency of progesterone. Excess estrogen causes symptoms such as irritability, anxiety, bloating, weight gain, breast tenderness, mild depression, and increased cramping. The opposite ratio-excess progesterone-may also occur. Excess progesterone acts as a depressant and can result in symptoms such as mild depression, decreased libido, fatigue, sedation, and water retention.

Estrogen and Brain Chemistry
Neurotransmitters are chemical agents released by nerve cells that convey "messages" between brain cells and are linked to our brain function and moods. Examples of neurotransmitters include serotonin, dopamine, and adrenaline. Variations in the amount of transmitters released of these neurotransmitters can alter our mental state; for example fluctuations in dopamine and serotonin levels influence mood and appetite. For women, variations in estrogen levels may affect the synthesis of these neurotransmitters and thus how our brain functions. Dopamine is particularly important as it is responsible for creating relaxation and mental alertness. It also lowers a hormone called prolactin, important during our second half of the cycle, influencing adrenal glands and kidneys and reducing water retention. The theory suggests the following: too much estrogen can lead to a related elevation of adrenaline (which can cause hyperactivity) coupled with a drop in dopamine (which is necessary for relaxation). This results in increased anxiety, irritability, tension, and mood swings.

Prostaglandins
This is the theory for menstrual cramps. Prostaglandins (PG) are hormone-like substances that are found in virtually all cells of the body. They function as mediators for a variety of physiological functions, including immunity from disease, inflammation of smooth muscles and dilation of blood vessels. Menstrual cramps are believed to be caused by an increased secretion of prostaglandins, primarily from the uterus. This can increase inflammation of the tissues as well as the strength of the contraction of smooth muscle. (the uterus is made up of smooth muscle)

Nutrition & Diet
Whether or not you suffer from PMS may be greatly influenced by your diet. Women with PMS generally consume more refined sugar, refined carbohydrates, sodium and dairy products, and fewer foods containing B vitamins, iron, zinc, and manganese. Nutritional factors may also play a significant role in the production and metabolism of hormones. As an example, estrogen is not only produced in the ovaries but can also be released in the intestinal tract. Estrogen is also conjugated in the liver. To maintain normal estrogen levels, it is important that food does not take too long to travel through the intestinal tract; this is one of the reasons a high in fiber intake is often recommended for women experiencing PMS. Diets high in protein and animal fats should also be moderate to control PMS. Both can result in the recycling of estrogen, thus adding too much to the body.

Psychological
Prolonged levels of stress have been shown to exacerbate PMS by affecting sex hormone production -- primarily by chemical fluctuations in the brain. The higher brain centers that coordinate our physical and emotional existence, no doubt affect the hypothalamic function. The hypothalamus as explained in Menstrual cycle explained plays an important role orchestrating our sex hormones. Acute stress is believed to inhibit proper functioning of the hypothalamus and explains the temporary loss of menstruation that may follow great emotional turmoil. In addition, prolonged feelings of stress stimulate other hormones, including those in the adrenal and thyroid glands, also known to interfere with sex hormones. Stress can be mitigated by both exercise and meditation which are proven to improve circulation as well as increase endorphin and neurotransmitter levels. "Fulfilling" social interactions and/or environment that is supportive can also reduce stress. As suggested, all of these can help dissipate stress and therefore make us less susceptible to mood fluctuations.