Can you write down everything you ate, drank, medicines, everywhere you went, etc. for that day and the day before? It might take a few days to remember everything, but there had to be an allergy trigger.
A trigger can be something that you've had previously. And the reaction can come as late as 24 hours after exposure. I once took a puff from an albuterol inhaler, with I'd had a couple of times before, and 8 hours later had intractable tachycardia. Migraine sufferers who have sulfite sensitivity can react the next day.