What can cause random allergic reactions to diet coke?

Allergy: Define or describe allergy. Many times what you think is allergy, is a side effect, not an allergy. Allergies usually are not random, they are persistent . Allergy is a reaction of itchiness, rash, hives, swollen lips/tongue or respiratory distress/ death in severe cases.

Not allergy: Allergic reactions are not random. If you're having an allergic reaction to a component of diet coke, you will suffer the reaction whenever you drink it. Manufactured sodas are very consistent, so if you're having some sort of reaction some of the times you drink it, you're reacting to something else - you're just associating it with diet coke. Your doctor or allergist can help you sort this out.