What does low energy without obvious cause mean?

I was at the doctors with my mom and I can usually stand for 2-3 hours while waiting for my appointment but today even sitting in a chair and staring at the ceiling made me tired, I had a miserable look on my face because I was tired, I think it’s because my lunch was different from normal: I had a fruit, a vegetable, a bowl of rice, and around 2 or 3 tablespoons of meat, and my lunches are usually full of proteins like noodles or rice or beef chunks, I usually have over 900 calories of rice or noodle for each meal, or maybe it’s weight loss from previous days.

What does low energy without obvious cause mean?
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