Used to get these like crazy. It can be from certain physical activities, poor diet, dehydration etc. My advice would be drink plenty of water, eat Potassium rich foods (bananas, sweet potatoes) and consider magnesium, either by oral suplement or topical (Epsom salts in bath or a product like theraworx directly applied to calf muscles).

My leg cramps completely stopped after I started using the topical magnesium stuff applied every few days.