Cramps at any time, or just while training?
Could be electrolyte balance. Make sure to get your salts and fluids.
Either way, magnesium deficiency is the main reason why the muscles can not relax. It's very difficult to correctly measure too, as only 1% is in the blood.
Supping 450-600mgs of magnesium citrate a day would be worth a shot, it isn't expensive.
Next is vitamin E, just make sure it isn't synthetic. Synthetic vitamin E actually drains the natural storage, and studies have shown it not to be beneficial at all.
Third is calcium, but most people are not calcium deficient. However many people are deficient in the nutrients that help PLACING calcium, which are magnesium and vitamin K2.
Hope this helps.