i haven't got time to research at the moment but i have a theory :^}. acid reflux (during fasting) may have something to do with the liver's circadian rhythms. apparently it signals the gallbladder to prepare to release bile just prior to each regular meal time and it may take a while for this to completely stop. this 'maybe' what causes the nausea around the 4-5th days when the stomachs mucosal lining has thinned.
sudden physical activity later on in the fast might also nudge the liver into releasing/leaking a small amount of bile in unison with the hunger hormones and changes in glucose levels the activity may have caused, bringing on the occasional bouts of nausea some experience weeks into the fast. just a theory mind you :-)
all the best
also since bile is sent out to break down fats and since fasting puts the body into fat burning mode, it may take a while for the liver to register that all the new fat burning signals from the cells are not from meals. the body is brilliant but it can't change as fast as our minds.