what are you developing an allergy too? A main ingredient or an ingredient that is sometimes used?
As far as a backup plan goes I'm not sure what other skills you have but the one thing I made sure I had when graduating college was a business degree because in the worse case scenario I could always go into management as managers are always needed. I'm sure though with the catering business you are in you already possess management skills with running a business and managing employees and maybe you should just update the resume to reflect that.
What things interest you? Could you possibly start taking classes in those areas? Just an idea.
Edited to add: Would allergy medicine or shots help at all? I'm not familar with either but if you love your job would it be worth it to look into?
Have you been to a doctor about this? I don't have much advice about a back-up plan but if you haven't consulted a doctor I really think you should get yourself to an allergist ASAP to determine if this is an actual allergy or just an intolerance. I don't want to scare you but some food allergies cause anaphylactic reactions which can be fatal and unfortunately there aren't any cures, only emergency treatment options if you do come in contact wth the allergen.
I think Aurora's idea of jumping into management is a good idea.
Do you have other food/catering related interests? Nutrition? What about event planning? My sister has a background in baking and is doing some kind of online certification program in event planning and she just got a job at a fancy private club where she will help put parties together but won't actually have anything to do with preparation of the food.