Post-viral syndrome is a thing common to most (all?) viral infections. For example, I got pretty sick in 2014 with some strange illness, maybe it was a bad flu or perhaps mono. Then for the next few months it'd seemingly resurface and cause little flare-ups of fatigue, nausea and a vague malaise. Just as it seemed to be fading I went on a lengthy post-college trip I around Asia where I'd spend multiple days a week feeling completely at ease hiking all day and partying all night but also had completely random days where I swore I felt like I was coming down with a flu and the littlest things would get my heart to race. When I came home the docs said it seemed like some sort of post-viral reaction then it resolved itself shortly thereafter. Covid doesn't seem to have been special other than that it gets tons of scrutiny and everyone got to experience it at once. Sometimes an overactive immune system will react in a counterproductive way and take a long time to get back to baseline, much like an army regiment that never got word that the war is over.