What kind of food do you eat? Regular food? Low carb?
You could be having a carb crash during fasting times.
I eat low carb, so I don’t have an adjustment during fasting times.
Also, how long are you fasting? If it’s long … are you taking electrolytes?
Electrolytes help me a bunch. I work out after my 13 hour Night Shift. I usually drink electrolytes during my workout, but before that I drink some black coffee and that’s enough to power me through 45 minute strength training.