Some time have cramp in my leg when i am having intercourse in missionry position, why this happensi?

You need to stretch.: Intercourse is an athletic activity, and muscles that are not stretched in advance may spasm and cramp. Perhaps massage and stretch your legs first, or suggest it to your partner. One of the best nutritional supplements to try is calcium lactate an hour before. Lastly, this will be easily aggravated by soda and smoking.

Poor circulation: That position may hamper blood flow or involve excessive muscle tention causing cramping. Recommend you try other positions like being on top, side or rear entry. Perhaps having intercourse in a semi-standing position would be helpful. Changing positions can have the added benefit of making sex more fun.