My son had stomach cramps all school term when he was younger and came to find out it was nerves. He never let on that he was anxious about school but once school was out the pains went away.
Also, a friend has celiac disease and there isn't a lot known about this but it does have similiar symptoms as IBS. You may also want to google this for more information. It's an allergy to certain foods. His was wheat and sugar.
I have IBS myself and now that I look back I realize that the symptoms began at around the age of 13.
I will tell you I had IBS ever since I was a kid. I remember crying of cramping, pain, and vomiting all the time as a young girl. It wasn't until I was 20 that I figured it out that I had IBS. I am able to manage it with my diet so I would strongly reccomend reading some websited on recommended foods for IBS. Stay away from beef, canned goods, acidic foods, raw veggies and fruits, fatty foods, candy, cakes, and so many more. Read about it and try easier foods for a while with your daughter. If only my parents asked more questions when I was a little girl.
Good Luck