Hip / Back Problems causing knee pain

Surprisingly it is quite common for certain types of back (spinal nerve irritation or compression) and hip (arthritis) problems to cause referred pain to the knee with no obvious back or hip pain.
This will only be able to be sorted out by a careful examination by an experienced medical or physiotherapy person. Spinal X-rays and other tests will often be necessary.
Treatment is directed at the hip or back problem of course and will often involve hip stretches and exercises or spinal manipulative therapy, which is best supervised by a properly trained and qualified spinal manipulative therapist or chiropractor.