I had severe gastrointestinal cramping and spent a lot of time on the toilet. I'll spare you further details.
I thought it was because I'd failed to notice the milk I'd put on my cereal had gone bad. So I threw away the rest of the milk, even though it smelled okay.
Two days later it happened again, and I hadn't had any milk.
I realized the culprit had to be something I'd eaten both days, and only on those days. Something I hadn't had before.
Ah-HA! It was the chocolate peanut clusters I'd gotten from a bulk bin at the grocery. They must have been contaminated with something. Won't be doing that again, even though they were mighty tasty.