Bones are the least of your worries if you are low on calcium. It is needed for many more important functions than supporting your frame such as heart function, muscle contraction, transmission of nerve impulses, blood clotting, regulating your metabolism, regulating hormones, and more. 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. Here is more info on PMS, calcium and calcium supplementation