How do I know it was the popcorn that was causing my abdominal pain?...
It got pretty obvious after a while. Popcorn is more of a snack than a meal, so I wasn't eating other foods with it. That means I couldn't put the blame on something else.
Here's what used to happen when I was still eating popcorn:
Day One: Eat popcorn. Yum! Twinge of pain in right side of lower abdomen.
Day Two: Popcorn on Day One tasted so good, eat more. Ouch—worse feeling in exact same place in abdomen as on Day One, like someone is poking their finger into my intestine.
Day Three: Give in to a little lower abdominal pain? Ha! Eat more popcorn. OWWWW!! This time it really hurts. Gee, maybe I should stop (duh...)
So I did, and I've never had the problem since. (Except for that one other time when I thought it might be okay because I hadn't eaten popcorn in such a long time. It wasn't.)
Why would popcorn cause this pain?
I don't know a scientific answer but I do know that I'm grain-sensitive. I react to all the grains that are in the grass family (cereal grains): wheat, rye, oats, spelt, kamut, teff, bulghur, barley and, of course, corn. So that could be the reason.
Or maybe it's because corn is a high mould grain and my system reacted to the mould. I really don't care what the reason is. If staying off corn keeps me from being stabbed in the gut, that's all that really matters.
BTW, I avoid all corn now—on the cob, corn chips, tortillas, high-fructose corn syrup, cornmeal, corn bread, dextrose, luncheon meats, and anything else that might contain corn.
What foods do you react to? Let me know.