What Causes Intermittent Lower Left Abdominal Pain Radiating Towards Back?

I have had lower left abdominal pain for the past 3 weeks. The pain is located below the belly button on the left side and radiates to the back. It is not a constant pain. Sometimes it is a dull ache and sometimes its a stabbing pain. Sometimes there is no pain at all. What could it be. I have gone for both pelvic and abdominal sonograms. Can it be trapped gas or gas that accumulates in my intestine.