It is called keto flu, iirc. As they say, your body undergoes a metabolic transformation (it knows how to do it from evolution/genes) when you miss carbs and sugar for a while. When I found out, it was so cool that our body can just reconfigure itself for a completely different diet. That is, if it is true, of course, because the entire keto-thing seems to be surrounded by some fog and doubt, for some reason.