Constantly feel exhausted?

So I'm 18, female, I'm healthy I exercise regularly, eat 3 meals a day, always have meat with dinner, I drink enough water, I sleep enough, But I constantly feel unwell in a way.

I just feel very tired, sometimes nauseous and have headaches, I can't concentrate, I get tired very easily, I feel horrendous if I don't get enough sleep, I prefer to sit down most of the time, just generally way more tired than I should be.

What does this sound like? What could be the problem/ be wrong with me?
Constantly feel exhausted?
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