R.A.
Poor thing! I would take her to her pediatrician, first. She might be more comfortable with a familiar face. If needed, I would ask the pediatrician ( who knows your daughter best) about a referral to a OBGYN, and who she might recommend. I would then prep her for that appointment, and ease her into it. If the pain is near her ovaries, they might perform an ultrasound, or something similar.
It could just be a cramp, or that she doesn't stretch enough before she runs or plays a sport. As well as dehydration.