Can Any One Explain This?

My 10 year old, Imelda gets a stomach ache every time she has a soda lately (pop or Coke for the rest of you, just to eliminate arguments). Well, almost.

See it turns out that she can drink Mexican sodas all night long (Which is NOT allowed). The Mexican sodas, Jarritos for example, are made with cane sugar. Not the high fructose corn syrup that American sodas use.

Has anyone else out there experienced stomach trouble from this stuff? Or is it just a coincidence?