@confetti11 - actually I seem to have a pretty good handle on why some days are good and some are bad. The causes range from PEM, being sick (usually with sinus problems), depleted adrenals (often after some type of stressor), low potassium (which I had off and on for years but only identified 7 years ago), depleted phosphorous and herxheimer reaction - all of these things cause fatigue. Also sometimes I will react badly to a new supplement (e.g., it may make my cortisol go too low). I've often thought we have to be detectives with this illness.
Through a lot of reading and also muscle testing done by my chiropractor and by myself, I can usually identify what is going on and then take steps to remedy it - e.g., add in potassium or V8, drink kefir for phosphorous, or increase my adrenal glandular if need be. These are all things I have learned about (slowly) over the years. I have also often thought I would be lost without muscle testing. It's cleared so many mysteries, I cannot imagine dealing with this illness without it.
If anyone is interested in muscle testing, I strongly urge them to first see a chiropractor or other trained practitioner so they can see and experience how it works. I've gotten pretty good at identifying symptoms, but sometimes I'm not sure what something is and will do muscle testing but then will see the chiropractor to see if we agree, and most often we do. A good place to start in locating a chiro who does muscle testing is the Standard Process website - they have a search feature for practitioners who use their products, which are very often used by chiropractors, etc.