need help about the diagnosis of this patient

Hi I am a 3rd year bachelors student in physiotherapy. There is a patient I hv seen recently who has following complaints:
pain starts around the rigth SI joint after long time bending work and spreads along the right gluteii... as the person tries to straighten up she feels greater pain. She says that if she rotates her hip in all directions for sometime, she hears and feels a snap around the right SI joint and the pain disappears. But such a pain rises again after any further long time bending activity.
history:
there is no history of trauma, she is 21 years old, a student in college, does no job, no medical problems.
observation:
her right piriformis is tight, slightly hypomobile right SI joint, normal sensations, no major motor loss

currently i hv put her on piriformis stretching n back care... but i cant make out wat is the cause of that snapping sound. plz help me.