It just shows how different we all are. in the early days I did 2 exercise programs. The first was a classic GET where one increased exercise slowly over time. In those days it was a increase activity regardless of how one feels and this was under the guise of the psych/physio lobby. Then my second was after they discovered (and with another hospital) how much damage they caused to patients and was an increase exercise with limits of how one reacted. Both were a disaster for me, the first was much worse than the second.
Then even after that, whenever I had a "good" (i.e. feeling different than normal but still nothing near well) I would always test my limits my slow exercise. I tried tai chi, yoga, lifting small weights, walking, slow swimming and other things. It was the same each time. There was a ceiling and when I reached that, bang, it was an immediate return to being largely bedbound again.
Most long term ME patients I know are like this and I've known people personally and met them at local groups who have been sick like me for decades. In my experience this "feeling better" raises enormous hope and longing in the severely and long term sick. I'd say from my first hand experience of knowing long term, acute onset, viral-type ME survivors that this would be typical.
Very frustrating. I tell myself that if I ever feel "better" again, the next time, I'll not do this. I'll try something different, an activity that is not exercise related, liked try to learn a new skill.