What can cause an allergic reaction?

I went to the doctor today and found out that I am breaking out in a major rash and getting swollen lips because I came in contact with something I am allergic to. She saw stuff on the back of my throat so she thinks it was something I ingested. But me or my mom can't figure it out. All summer long I have been sitting in my room playing xbox. All I can remember eating is taco bell, little ceasers ( its a pizza place idk if its in other states) mc donalds and subway. None of these foods are new to me and I haven't ever had an allergy attack before. Is it possible to develop allergies over time? One new thing I recall doing was drinking out of those thermal coffee cups that is made of stainless steel. But I use stainless steel forks and spoons all the time so that couldn't have caused it right?What can cause an allergic reaction?
you may be allergic to wheat as its in most of the foods you eat, and an extremely common allergen, or maybe dairy, but i would bet my money you have developed an allergy to some wheat protein, maybe gluten

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