Sorry you didn't get any answers yet!! I am hoping you've gotten your daughter to the doctor. The one leg getting smaller is a little concerning! It could just be growing pains for having pain behind the knee. But it also can be a sign of other things that you would need to know about like juvenile arthritis, lupus, lyme disease, etc. The area you describe though sounds like her hamstring? This is the muscle on the back of the thigh. Again, get her checked out. Could be a pull. Cold and warm on the area? Icy hot on the area? gentle stretching? But I would only do these things after having her seen by a doctor. Let me know how she is doing, okay?