Make sure you're drinking plenty of water! I go off plan weekly & occasionally I do get mild headaches the following day, but my low carb days aren't Atkins-low so I can't necessarily relate to the extreme nature of your headaches. However, this morning I did observe that starting out my day hydrating right away (before eating) helped it go away within an hour or two. I'd be willing to bet the sodium in what you ate played more of a role in your symptoms than the carbs...in my case excessive sodium on carb-up days spikes my blood pressure, sometimes high enough to cause headaches. Drink that water girl!