Going lower carb takes some adjusting to. Your body is used to swimming in glucose and now it’s not. You will get used to using (body or eaten) fat for energy but it takes a while.
What can often happen in the early days is you become dehydrated (carbs hold water) or you drink more and flush out your electrolytes. Add to that you are probably (hopefull) eating less processed foods so you dont then get the electrolytes they contain. make an effort to eat foods rich in potassium mag and sodium. If you can’t try a supplement til you can. People can feel hugely better within hours if this is their issue. It seems to get easier and less of an issue as you adapt over time. Other wise vitamins and minerals should be as needed rather than making expensive pee for things you don’t need.
A comprehensive check for common tiredness causes should include the b vitamins especially b12 and folate, vit d, iron and ferritin, and thyroid panel as well as fbc. Also be aware the lab reference range is what’s commonly seen in those tested. It is not the same as normal and definitely not the same as optimal. Remember a lot of those being tested are sick in some way and they will squew the range of what’s typically seen. Eg a huge minority of adult women are iron deficient even if not fully anaemic so levels are often seen way lower than they should be. B12 levels in the U.K. are set very low (160) compared to other places in the world (Japan 500) and you can be symptomatic way before you get that low.