Buckwheat is not a grain: (I am such a picky nerd); but you might be allergic to it.
from Wikipedia–
Despite the name, buckwheat is not related to wheat, as it is not a grass. Instead, buckwheat is related to sorrel, knotweed, and rhubarb. Because its seeds are eaten and rich in complex carbohydrates, it is referred to as a pseudocereal.
