My 4 Year Old Son Starts Crying Loudly After He

My 4 year old son starts crying loudly after he is sitting on the potty for more than 3 minutes. He says his right leg hurts so bad. I tried to move it a little and he screamed. This happens every time now. He is little and his legs do hang down without touching the floor. I figured maybe that was why but I'm not sure. He points to the area of his thigh. Only the right leg too. He never complains of it hurting him anywhere else but the potty. And he is a very active child who runs, jumps and climbs everything. What can this be?