What Causes Severe Muscle Cramps Between The Hips?

Hi, I am a 72 yr old woman, who up till last year had very few aches and pains. I now have a very painful back muscle cramp after overdoing gardening. I have a warm bath before bedtime, and wake 3 hours later with what feels like a total cramp going on between my hips. I do some leg stretches etc, and manage to stand up, and walk a lot before it eases off. I have had spinal Xrays, and no damage there. I am now wondering if is diet. I am 178cms,and 85kg s. I work as a Bus Driver, as well as doing gardening and housework. Could this be a mineral deficiency?? As the day goes on, my back tension eases. Maybe I should lose weight . Any suggestion would be gratefully accepted. Helen.