Is the pain constant or mid-cycle? If it's mid-cycle, it could be mittleschmertz, which is perfectly normal and harmless. Anything serious should have shown up on ultrasound or during your pelvic. I would seek a second opinion if the pain is more than mid-cycle.
How old are you? Could be a lot of different things. I was having the same problem an every test kept coming back normal. Check webmd that will give you a start on what could be. Could also help you doctor rule things out.