Could be die off, that's for sure. It could also be your body getting used to the strict diet. How strict are you being? It's pretty tough on your body to go straight into starving it of carbohydrates and everything else that it's been guzzling beforehand. If you are off carbs, try a bit of brown rice now and again as it can be terribly bad for your body just to go straight into protein only and you need a bit of energy. Make sure you're eating all the vitamins and nutrients you can and if not, that you are taking vitamins as supplements- it can be easy to get malnourished.