What Causes Cramps Back Of Thigh?

71 years old 57" 200lbs and losing from 223 lbs. walk 2 miles fast pace 7 days a week last 5 years average time 37 min. about a week ago got pain behind knee cap then cramps back of thigh .walking or standing 10 min or more extream charlie horse pain. pain goes away when i sit 5-7 min, then pain comes back when i stand. no swelling no spot tenderness.