A word about carb flu (also known as Keto flu)
If you haven't low carbed before (and possibly even if you have), you won't have experienced carb flu.
Not everybody suffers from this, but be prepared for it. It usually strikes around day 3, and lasts for 3 or 4 days.
Here's a bit from The Diet Doctor about it:
The cause
The keto flu is caused by your body’s transition from burning sugar to burning fat for most of its energy needs.
Switching from a high-carb diet to a very-low-carb diet lowers insulin levels in your body. This is not only healthy but also one of the primary goals of a ketogenic diet. When insulin levels are very low, your liver begins converting fat into ketones, which most of your cells can use in place of glucose. When your body is mainly using ketones and fat for energy, you’re in a state of ketosis.
However, it takes your brain and other organs some time to adapt to using this new fuel. When your insulin levels drop, your body responds by excreting more sodium in the urine, along with water. Because of this, you’ll probably find yourself urinating a lot more often in the first week or so of a keto diet.
This change is responsible for some of the rapid – and usually very welcome! – weight loss that happens in the early stages of a keto diet. However, losing a lot of water and sodium is responsible for many of the unpleasant symptoms of keto flu.
It’s well known that response to the keto transition is very individual. Some people may feel fine or slightly tired for a day or two after starting keto. At the other extreme, there are those who develop symptoms that strongly impact their ability to function for several days.
Read more about it on the diet doctor website