Tiredness is a symptom of just about every illness and condition out there.
As others have said adjusting to going low carb can be draining if it’s only started since the change of eating. Make sure you aren’t depriving yourself of energy by trying to do low fat as well as low carb. Also make sure you’re getting enough water and electrolytes (add some salt) as less processed food and higher fluid levels mean electrolytes are lowered and get ‘washed’ out more easily and can make you feel truly pants if they drop. Mostly magnesium and potassium.
If it still doesn’t go away check FBC, iron levels, not just Hemoglobin but also ferretin, also b12, vitamin d and thyroid levels. And get the actual number results not just “normal”. Too often you can be right at the lower level, or even a bit below, and it gets written off as ok. And that’s before we discuss if nhs levels are optimal or just typical of a general population that will also include sick people and whether you personally need to be at one end of the “normal range” and are in fact sat at the other far end but still “normal”