Leg pains

Zack has been having some severe leg pains the past 5 days or so. Started off just an annoying pain but now he is having hysterical crying pains. Kinda scary. His pediatrician thinks it may be cramps but I'm just not so sure. My gut thinks its more nerve than anything. It only happens when he is sitting on a chair or the toilet, not when he is running, playing sports, sitting on the floor, only when he is sitting on a hard chair. I got a call from school that he was in speech and all of a sudden started crying doubled over in the chair. He got up and it was done. Very strange. Could it be a side effect from meds? I don't know!