Hi, Michelle!
I've read all of the responses to your post, and it sounds like you are getting some great advice about things to check into. I have a 10 year old daughter, and we experience the same issues! One thing I wanted to mention that I haven't read in the other posts is the possibility of chronic constipation. This is primarily what we deal with, and there are a couple of main culprits for it...at least in our case.
One cause of constipation can be prescription medications, which my daughter is on (for petit mal epilepsy and ADD); medications often list constipation as a common side effect. Another cause can be diet. A diet high in proteins (derived from animal proteins such as meats, poultry, fish, eggs, cheese, milk and other dairy products, as well as plant proteins such as nuts and beans) can often cause constipation, especially if the protein content outweighs the fiber content (derived from whole grains and fresh fruits and vegetables). Many of us, unless we are consciously consuming a diet in which protein plays the smallest comparative role, end up consuming protein in the largest amounts, followed by grains and fruits and vegetables, even though we believe we are "eating healthy."
In our case, since we can't avoid the medications, and since my daughter is a typical child in that she is a fairly picky eater and not necessarily concerned with the long lasting ramifications of the standard American diet, we use Miralax powder as an additive to her skim milk once a day. (The amount, I think, is 17 grams.) We have been doing this for some time now on the advice of our pediatrician. The powder is tasteless, although it does leave a bit of a residue behind in the glass. When she takes it, she has more and softer bowel movements and far fewer stomach complaints than when she balks and goes for a few days without taking it; then, we're back to the frequent stomach upset and pain.
Hope this helps, and best of luck to you and your daughter!
~S.