Magnesium is very effective in reducing menstrual cramps. It is a natural muscle relaxant. Use an acidified form such as magnesium malate or citrate, 300mg 2 to 3 times daily. Do not take it with calcium since calcium is a muscle contractor and antagonizes the magnesium.
Cramp bark can also be used to relax the uterine muscles.
If your cramps are also associated with heavy bleeding then endometriosis may be the problem and that will have to be addressed differently.