Believe it or not, it could be growing pains. My daughter experienced this, ( growing pains), from the time she was 4 until 8 years old. It usually surfaces while sleeping, or right when they get up. I was really taken by surprised, because I thought "growing pains" was an emotional term, when it in reaity, it is definitely physical, and definitely painful. The child complains that it is in their leg, but, supposedly it forms moreso from their back and their hip. It just means that they are growing. I used to give my daughter some tylenol, and for the years she experienced this, we called it "leg medicine"! I thought, at first, my daughter was imitating her father, complaining like he does, with leg and knee pain, because I never heard of the reality of this. It is real, and so is the pain. Best of Luck!