Happens to me as well, just like that. Sometimes I could eat a biscuit and have no trouble, then I'd eat cake and be laid out for hours with those very symptoms.
When you go off them, and try again, I think it's a shock to the digestive system to suddenly introduce something it considers toxic once more.
Many people have these symptoms, but attribute them to something else, like bad seafood, as you did. I finally decided no more and haven't had any more distress. I don't even want to see wheat again for the rest of my life.
Start weight: 250 - 06/2009
Current weight: 199