I get them too... Usually after eating grapes or satsumas. Doesn't matter if they're washed, or even if they're organic... Within a few minutes of eating either of these fruits there it is... Leering at me and no matter what I do I swear it laughs as it gets bigger and watches me frantically try to stop it in it's tracks :(
From the acid in the fruit
I get canker sores when I eat pineapple, grapes, and strawberries, pizza ( tomato sauce), spaghetti (sauce again). Anything acidic can trigger a canker sore in my mouth, usually inside my lower lip.