--the usual figure given is that up to 20% of individuals with biopsy-proven Celiac dysfunction of the intestinal villi have negative Celiac panel bloodwork. So it's possible that she does have that issue, or at least gluten sensitivity, if not full-blown celiac.

Other people more expert than I will come respond to you, but in the meantime take a good look through The Gluten File (at the top of this board, in the Useful Websites section) and go through the diagnostic section--lots of good info there on this topic.