I'm not sure that an allergy can happen overnight. I think signs of it may be happening and you don't notice it. Any scientists out there?

While allergies don't usually instantly happen, they do come and go through life (outgrow or grow into them).  Usually when an allergy hits suddenly,  its due to a new substance you didn't know was there but that you have been allergic to all along.  For example, one can eat blueberries all their life then suddenly one day they become allergic to them.  While it may be the berry, it's more likely something that was added to the berries that wasn't there before (a different pesticide, an added ingredient to what you ate with them or put on them, etc.).
One can be allergic to, say for example, ragweed, but had never come into contact with it.  Then one day while they were eating or doing something, a truck that had come into contact to ragweed drives by.   The person may assume that what they were eating or doing was the culprit, when in fact it wasnt.