Hello and thank you for posting your medical question on E health Forum.
It is not possible to know the cause of her problem by the available history here.
A complete history and a physical examination would be needed to know the exact cause.
But the most probable causes could be gastritis, acid reflux, duodenal ulcers, enteritis, or a parasite infestation of her intestines.
You might consider to consult your doctor for proper diagnosis and treatment.
I hope this helps.