Dogs naturally develop allergies suddenly, especially to corn and grain because they aren't supposed to have it in their diet. Their bodies aren't made for it. Allergies are one of the top problems with dogs these days. So it's definitely not uncommon. I don't think a corn allergy would have anything to do with flea medicine at all... The only thing that flea meds could possibly do would be give a skin reaction and cause skin allergies maybe. It's probably just your luck that that all occurred at the same time. Or maybe they had a reaction to the flea medicine which made their immune response down slightly and now the true allergies to corn are coming through?