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It very well could be the grape juice. I'm sensitive to sulfites and react to grape juice because its high in sulfites. Alot of the gluten-free foods or healthier type products are sweetened with grape juice or other fruit juices. If you were sensitive to sulfites you would also react to red wine, dried fruits and vegetables, dried coconut, etc.
Anything with these ingredients contains sulfites

Sodium Sulfite, Sodium Bisulfite, Sodium Metabisulfite,
Sulfur Dioxide Gas

Depending on the persons sensitivity they may react to only those foods which are very high in sulfites (i.e. grape juice) or they may react to nearly ALL processed foods, as sulfites are in nearly everything.

It might be something to look into if you notice a pattern.

Definately dont eat Kellogg's Corn Flakes...try another brand that is gluten-free but not sweetened with fruit juice and see if it helps.