Just chiming in to say that we have the same problem here with my daughter. Some days she's okay and seems to tolerate things fine. Then the next time we're like "oops, steak gives you a tummy ache so no more steak". We go thru this constantly.
Sickandlucky, I can say tho that we have found that those same exact foods (sugars, fats, fried things) are almost a guaranteed stomach pain trigger. Not gluten, sometimes dairy but only if there's LOTS of it (ie, milkshake).
This is so hard to figure out. She seems to do best on a Low residue, Low fiber diet. So much less tummy pain this way, but also so limiting.