Strain, bruising, arthritis or growing pains are possibilities. Sometimes pain can be from undertermined causes and clears up by itself. Persistant pain should be evaluated by a doctor.
A tight psoas muscle can pull the whole pelvis, which the hip joint is attached to. The imbalance can cause the structure to put pressure on the nerves.
Assuming you haven't recently been in a car accident or a serious fall, you might be suffering from sciatic nerve pain, which begins in the hip or buttock.
