Iliotibial band syndrome is a common cause of pain from the knee, this is normally caused by a muscle imbalance from exercise done the wrong way or too much stress on particular muscle groups while recovering from other injuries.
Pain from this area is basically caused by rubbing of this band, which is like hard plastic, against the joint that it runs over.

Pain can also come from the outside of the hip area.

The stretch:

Starting position:

Stand side on to a wall with the arm closest to the wall outstretched and on the wall.
Cross the leg furthest from the wall behind the other leg.
Anchor the rear foot and gradually put more weight on the hand that is against the wall.
Move your hips away from the wall.
You will feel a stretch down the outside of the leg furthest from the wall from the hip region down to the knee.
Hold in this position for about 5 seconds.
repeat 10 times.