I guessed that I might have a hormone imbalance of some kind, perhaps with my thyroid or testosterone (and, no, despite appearances, not overburdened with an excess of it). But it could have been something far more wicked, such as CFIDS/ME (Chronic Fatigue Immune Dysfunction Syndrome, also known as Myalgic Encephalomyelitis), which I was exposed to for years. I've seen what CFIDS does to someone, and it's not nice.
As it turns out, it was neither hormonal nor CFIDS, just some passing virus, like catching a severe flu with no muscle aches, fever, or rumbly tumbly. I hadn't known that it was possible, for such a minor reason, to feel as if one's veins were filled with molasses and one's eyelids weighed 17 pounds. Each.
But it's over and no reason it should ever come back.