Dear Abraham

You sound like a pretty chipper person, but that is hard to “analyze” from this one post. What I can read is that your fatigue comes from nowhere, or not from things you could pinpoint exactly. So maybe go see another doctor, check your Vitamin B12 and other things until it’s clear that your problems are not physical.
If there’s nothing to find, it is possible that it comes from a mild depression, or depressed phase. I myself have experienced this when I had depression. But it wasn’t just a few days, it was more and more and at some point all the time. So seeing a psychologist might be a good thing. Maybe there is a traumatic event or a stress situation in your life that you are not dealing with? That can cause such issues.

I wish you all the best!