The longest muscle in the body, Sartorius is a hip flexor and an external hip rotator.
It originates at the front of the pelvis (at the Anterior Iliac Spine) and inserts on the medial knee - on the tibia in the lower leg. (see image)
Translated as 'taylor's muscle,' Sartorius is shortest (contracted) when we sit in the cross-legged position. It's one of the many hip flexors that most people consider a 'single muscle' as in "My hip flexor is really tight."
Ask your therapist to check if your Sartorius is tight, or contains any trigger points.