Feeling tired after eating low-carb for only a couple days is an extremely well-known temporary side effect, so I'm surprised you weren't aware of it if you were actually intentionally going on a low-carb diet.

I wasn't; I spent a few weeks in a country whose diet includes much more meat and less grains than mine. And I meant tired as in tired (i.e. bored) of always eating meat, not physically tired.