Every body is different and, while I never experienced leg cramps with my period, that doesn't mean it's abnormal for you. I suspect that you're losing too much of a particular mineral with the blood loss, maybe magnesium? You might try taking a vitamin supplement designed for women. They add in extra stuff to replace what women lose during a menstrual cycle.
My doctor told me that I am anemic and lose a lot of iron during my period which causes leg cramps, due to the weakness. It might be something different for you. You should talk to your doctor to find out.