Calcium is needed for many important functions than bone health, such as heart function, muscle contraction, transmission of nerve impulses, blood clotting, regulating your metabolism, regulating hormones, and more. Being calcium defiecient can affect all of these functions,so it is not a big stretch to think that calcium may be involved in the irritability, mood swings, pain and cramps associated with PMS. So test it out, try taking a calcium supplement to get your calcium levels where it should be and see if you feel better during PMS than you usually would.