The most common cause of stomache complaint in children is Functional. Functional means that there is no organic cause. But this diagnosis should be made after excluding all other major Illness.I hope as you have told,your child has been examined and managed by Pediatrician
all these illnesses have been ruled out.
You should improve bowel habits of your child by
1. Give her plenty of water.
2. Adding more roughage in diet
3. Try to remove any kind of worry in her by giving her more love
4. Avoid milk in her her diet for at least some time to rule out any kind of milk allergy
5. You should Deworm your child.
6. Avoid her to give spicy and fast food.
Wishing early recovery for your child.