I have a mystery food allergy that leads to severe bloating and diarrhea. i usually take an antihistamine & am fine. had a bad reaction, felt faint, bright red blood in stool. should i be worried?

Start a food diary: Though it is time consuming, you may wish to start a food diary. Begin with simple foods (only!) to which you KNOW you don’t react. Then add a new food ever 3 or 4 days and watch for an allergic reaction. Note on paper or smart phone (there’s a food allergy app for this) no, mild, or bad reaction. This takes time and diligence, but you should be able to find out which foods to avoid.