Here is something else I was wondering if might be causing your hip pain.

http://www.coloradocenter4pt.com/neuromuscular.html if you scroll about half way down you will see a diagram of where the Piriformis muscle can cause pain and areas where trigger points are located. A tennis ball is a handy way of working those trigger points. A good physiotherapist or massage therapist can often easily determine if this is at least the partial cause of your pain.