Doesn’t even have to be an illness you remember. A mild Corona case, a flu, EBV could all lead to this.

What happens if you overexert yourself? Do you feel sick 0-3 days later with flu-like symptoms? Try to look out for that. That’s the most prevalent symptom of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome/Long Covid. The worst about it is that the really bad symptoms turn up with a delay from the trigger, making it harder to notice.

Hope that’s not the case and that you manage to find out what it is.