What Causes Pain In Lower Abdomen And Back?

My 19 year old daughter has been having lower abdomial pain an on both sides fo 3 weeks now. She had a CT scan and it didn t show anything and she still has the pain but it s now in her lower back and between her shoulder blades an she stays nausea. I keep thinking gallbladder or gallstones, but then other things worry me too. What should we do now?