Pretty much all dietary allergies are to a protein, its usually specific to one protein IE lamb, beef, chicken etc, the problem is finding which one as most completes have a mix of different ingredients. The leading anti allergy diets tend to use "novel" proteins like feathers etc that the body won't have met before so they don't cause an allergic reaction.
Having said that as Byron notes above "stress" is one of the biggest causes of stomach upsets and often gets lost in the quest for a cause leading to random allergies being blamed without a definitive diagnosis. As you say, sounds lik