The severe headache the next day felt like what's called "keto flu" which is caused by an electrolyte imbalance. It's not really a flu but happens to many peopl

The severe headache the next day felt like what's called "keto flu" which is caused by an electrolyte imbalance. It's not really a flu but happens to many people when transitioning into keto if they don't take care to supplement electrolytes (primarily salt). The mechanism is well-understood. It happened to me when I first started keto because I foolishly ignored the warnings about supplementing electrolytes. The headaches and other keto flu symptoms usually start 24-36 hours after stopping carbs and can persist for a few days if not addressed with electrolytes. I haven't needed to supplement electrolytes since my first month of keto because I'm fully keto adapted. I think the episode where I stupidly 'overdosed' on sugar my metabolism was no longer adapted to handle, pulled me fully out of keto and caused an electrolyte imbalance similar to what I experienced early in keto.