It's true that a "syndrome", by definition, is a collection of symptoms that seem to co-occur. And it's also true that there are many possible causes of low energy.
But there seems to be one particular cause, not that anyone seems to know for sure what that is, that produces a set of symptoms that is more specific than generalized fatigue. The symptom sometimes called "post-exertional malaise" is, AFAIK, unique to this disease. One doesn't necessarily feel tired while exercising; instead, the tiredness (often accompanied by headaches and sometimes other symptoms) hits hours or even a day after the exercise.
So when the OP writes "Instead of trying to continually increase my exercise, I’d learned to focus on staying within my ever-changing limits", I know she has the same thing I do, because that's my experience exactly. Whatever you had, in contrast, sounds like it had some other cause.
But there seems to be one particular cause, not that anyone seems to know for sure what that is, that produces a set of symptoms that is more specific than generalized fatigue. The symptom sometimes called "post-exertional malaise" is, AFAIK, unique to this disease. One doesn't necessarily feel tired while exercising; instead, the tiredness (often accompanied by headaches and sometimes other symptoms) hits hours or even a day after the exercise.
So when the OP writes "Instead of trying to continually increase my exercise, I’d learned to focus on staying within my ever-changing limits", I know she has the same thing I do, because that's my experience exactly. Whatever you had, in contrast, sounds like it had some other cause.