My daughter (who is now 8) complained about stomach aches for probably two years before we figured out she was Lactose intolerant. We tried Tums, Rolaids, Pepto.....everything. Those things helped, but never "fixed" the problem.
Finally, last year we took her to the doctor because I was also worried about an ulcer (resulting from what we called a "nervous stomach") or food allergies. Her pediatrician suggested we take her off of dairy and that it might be Lactose intolerance....which no one in our immediate family has.
Within one week of banning dairy, she was "cured" of her stomach aches. She used to LIVE on string cheese, yogurt & milk! Now we both know that if she is going to be eating anything with dairy that she needs to take her Lactaid. We have the chewable Lactaid tablets and those really work!!!
Try the "no dairy" diet for a week and see what happens...also make sure you check the ingredients in packaged foods. A lot of them contain dairy even though you wouldn't think they would. Most of them list dairy under, or even within, the ingredients.
Hope this helps.....GOOD LUCK!! :)