Rectus femoris is the longest head of your quads. It's the only one of them that goes over two joints. (The hip and the knee).
A good stretch for the muscle is one where you flex your knee and extend your hip. Like this
. If you don't feel the stretch well, push you hip forward and flex you knee a bit more than on the picture.
What puzzles me is that Quadriceps isn't really working when you rotate your hip, aka turn your feet inward. The main muscles doing that function are Tensior Fascia latae, and Gluteus medius and minimus.
Is the pain similar when you're walking, and does it occur in any other movement? I wouldn't blame the lower back at first, but if there is a postural issue it could be possible.