Rash

I have developed a red rash around my mouth, which itches, hurts, burns and is very red and sore. Sort of like a case of severely chapped lips. I developed this for the first time after drinking a homemade smoothie (using only fruit). It starts to get better, but them something I eat triggers it again. I cannot pinpoint what is doing it. I think it might be an allergy to something they add to food, maybe a preservative. A few days ago I ate a salad and that made it worse. I think it might have something to do with food that is processed. This rash is very uncomfortable and I don't know what it is.  I had this same thing happen to me a couple of years ago. I put up with it for a few months and then set up an appointment with an allergist. By the time my appointment came around the rash went away. The allergist did the ***** test and found that I was not allergic to any type of food. He said without seeing the rash, he could not tell me what it was. Well, I went for two years with no rash and now all of a sudden it is back. I am not eating any differently than I ever have. Do you know what it could be and what I can do about it. Any help would be greatly appreciated.