Dear Jeanine
Yes, it is the puzzling and scary thing about our disease that we can feel pretty good one day, and completely "petered out" the next, sometimes with no particular pattern or explanation.
I will feel strong after a tough day and a short night, and will feel completely conked out after an easy day and restful night.
It is essential, however, to keep exercising, and maintain stamina and core strength. The worst one can do is to resign oneself to the wheelchair and immobility.