Food Allergy?

I think I might have a mild food allergy. I have no idea what to. The only time I have noticed it when I have eaten two different types of cookies. I ate them months apart. I looked on the ingredients of the first batch, but this second batch didn't have any. I have no clue what it could be. It doesn't happen for every cookie I eat. So it just has to be some kind of ingredient that isn't used all the time.