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?