Many possibilities. From a GI standpoint, there are many possibilities. These include irritable bowel syndrome, food intolerance, inflammatory bowel disease, even cancer. More information would enable a physician to narrow down the possibilities to ones that are most likely for you.
Food intolerance. As long as she's sure she isn't pregnant (which would require immediate medical attention), daily abdominal cramping can be caused by food intolerances such as lactose intolerance or celiac disease. Constipation and irritable bowel syndrome is another common problem. If this has been an issue for more than a few days, it should be checked out by a doctor.