My Sister Is Experiencing Stabbing Pain In Her Lower Back

My sister is experiencing stabbing pain in her lower back on the left side, just above the hip. She says it is not "back" related. Pain comes and goes in 15-20 minute increments and has become more frequent over the last several weeks. She has been diagnosed with IBS. She eats a very healthy diet and works out regularly (cardio, strength, yoga, etc.). Can you give me any sort of idea as to what might be going on?