I've read that modern "dwarf wheat" has an enzyme that interacts with the same neurotransmitters as opiates, prompting withdrawal symptoms if one doesn't have it again in a few days.

I've got strong source amnesia and am probably misremembering details: maybe it was from "Grain Brain"? [0]? At any rate, my experience has been pretty similar to yours.

[0] https://www.drperlmutter.com/about/grain-brain-by-david-perl...