sudden stiffness in the lower legs for short time

My daughter 8 years old .she was born as premature baby but her general health is o.k. For the last one week ,on atleast three occasions  ,particularly in the evenings ,she complained that she developed sttiffness in the legs suddenly and not able to lift her  legs and not able to walk for some time -for about 15 - 30minutes and manage to walk by dragging her feet  After 20 -30 minutes ,she  is able to move her legs normally  and walk normally and even jump .There is no swelling of the legs and no reddening except  she complained of pain below the calf musles and above the ankle .we  are not able to figure out the reason .There is no fever and there is no other health problem expect she uses the glass for reading .
apreciate  your advice on this .