That happened to me today. We were out running errands and, thanks to crappy planning on my part, ended up at a fast food restaurant for lunch.
It tasted crappy and then within half an hour I ended up with stomach pain that lasted through the afternoon. I used to have problems with IBS which has gotten a lot better lately, but apparently eating crap (after eating healthy for a long time) was enough to make it flare up again. Lesson learned the hard way for me, I guess.