Dehydration or low vitamins and minerals in your diet. It could also be "growing pains". Drink plenty of water (at least six 8 oz. A day) eat a diet rich with green vegetable (like spinach and broccoli - iceberg lettuce does NOT count) and fruit with potassium like bananas. Get plenty of rest (at least 8 hours) if they continue talk to your doctor. Good luck
You could be low on electrolytes. Especially calcium, magnesium, and/or potassium. You may be dehydrated or coming down with something. Best thing to do is go to your family doctor for a check-up and blood work before you try any treatments on your own.
Do you drink excessive amounts of alcohol? It sounds like your electrolytes are messed up. Take some vitamins. Muscle cramps can be caused by calcium or potassium deficiencies. Try eating a banana a day and drinking milk.