Agree with all the replies - in Gary Taubes book "the case for Keto" - he really stresses that there should be an honest conversation about the transition that you inevitably go through when you first cut carbs - it's basically withdrawal, like any form of withdrawal - and as long as you expect it, and understand what's going on, it shouldn't be a shock and you can see it for what it is.
But - ignoring the likelihood of having an initial period of feeling not great does make people feel suspicious "I thought I was going to feel great immediately and now I'm worried that all of this low-carb stuff is making me unwell..."
It's totally natural; so we should talk about it.
But yeah, replace the fluids and electrolytes (mainly salt - you will also find that all your natural "reduce salt consumption" expectations are on their head - I grind up about 1gm of salt - about 20 twists on the salt mill - very fine and take that with some apple cider vinegar and fizzy water every day.
It does pass fairly quickly though -