Allergy to what? Fever, diahreea and bad swollen lip and eye?

My 19 months old got all this in one night , we went to ER fast and after all tests and xrays , 2 days in IV -no answer! All tests were negative so we don't know what caused all this.It's scary, did anyone experienced someting like this? He was tested for common food allergies and all came back negative.Shouls I go to a allergen specialist? They gave me an EPI pen to have with me all the time just in case....i am going crazy...please share!