It can be very challenging to differentiate the cause of stomach upset initially. Generally speaking, food borne illnesses come on very quickly after eating, are very intense for a short period of time and then quickly resolve. More significant viral, bacterial or parasitic GI illnesses often come on gradually and may last for several days. GI infections have many similar symptoms regardless of cause and may include fever, nausea, with vomiting and/or diarrhea. Most mild foodborne illnesses do not have the feature of fever.
Food allergy typically presents with different symptoms including skin rash, tongue or lip swelling, difficulty swallowing and difficulty breathing. Simple food intolerance, such as, lactose intolerance, will have symptoms of gas, bloating and possibly mild isolated diarrhea. If you believe that you are having an allergic reaction to a food, try taking diphenhydramine and contact your primary care physician for follow-up. However, if you are having difficulty breathing or swallowing as a result of a possible food allergy you should seek care in a certified urgent care or emergency center.