It's not keto flu and it isn't a problem.

You started keto 3 weeks ago. This is the point where you're coming up on "fat adaptation" to where your body is now able to burn stored fat pretty efficiently for a fuel source.

Because of this, your appetite will decrease. It is normal. You don't have any problems as long as you don't eat all those carb heavy meals you're craving.

Perhaps it is the holidays that has you in this funk? You might not consciously be missing carbs, but subconsciously you might be and that's causing your cravings (you know, seeing you "skipped" the carb heavy holiday 3 days ago).

All I'm saying is we had a major holiday 3 days ago, and 3 days ago you realized you hate eating keto and crave carb heavy foods. It very well could be brought on by the fact you stayed strong during the holiday and mentally you're feeling cheated.

Anyways, that's just my 2 cents. There's nothing wrong with you, but you really should think about eating something small if that's all that you can safely consume without wanting to vomit.