Google, "allergic coke zero" and you'll get hits. Only problem is... substitute "roast chicken" for "coke zero" and you also get hits. It even works for, "watermelon" (although, apparently its a rare allergy) so... my guess is, we're not 'gonna know for sure but it could be you do have an allergy even though few others do. Lucky for you, it's as easy to stay away from Coke Zero as to stay clear of, e.g., camel dander.