The first few days of switching to a ketogenic diet sucks. You'll feel hunger pangs, cravings, grumpy and probably suffer flu-like symptoms. You may even feel light-headed, sometimes. There are a number of reasons for this, including changes in gut biota, as various colonies are "screaming at you" for food, but mainly as your body initially will be looking for a quick glucose energy source, before switching metabolic states and preferring ketones, instead. This takes some patience and discipline.
The beauty of a ketogenic diet is that you often don’t need to be too exact with your calories. You can just keep carbs low, fat high, and everything works itself out over time. If you want to figure out how much you should be eating(it’s worth knowing), then try our Keto Macro Calculator. It takes about 3 minutes to calculate and there is a how-to video explaining how to come up with an accurate calculation.