Since the symptoms go away with antihistamine, I'd suspect a simple allergy.
Have you had a skin prick test? A GI might not do that, but an allergist certainly will! It's possible to not react to the test!
Keep a food journal in case you react to other foods or things with which you come in contact. Since the corn cob leaves triggered a reaction, pay particular attention to corn-related things.
There are many lists online such as: www.cornallergens.com/list/corn-allergen-list.php