You need to consult with a specialist in allergies, and take the series
of tests he advises. Often there are a constellation
of foods/substances associated with a specific allergy.
Many catsups are colored with food dye, so that may be the issue. I would also write the manufacturer of the brand and tell them you had had a reaction.
The mechanism of these allergies are little understood. One theory is that there is a genetic inability to process certain chemical substances.