the mt dew could definitely be the culprit. Diet just means no sugar, it doesn't mean it does not have caffeine. Caffeine releases stored glucose from your liver which will definitely make your insulin go up. in essence it's like eating something sugary. Just check on caffeine on your computer, it will tell you what it does. Diet sprite and diet seven up contains no caffeine.
It's amazing that many low carb books will allow coffee and tea on these plans, when caffeine will sabotage the whole thing, unless you have very small amounts, like weak tea. hope this helps....?