This could be low carbs or low thyroid. If it continues, load up on a carb heavy meal before you begin and see if it still happens at the same time. E.g. if it’s happening now at 48 hours and you eat a lot of carbs before you start, then it happens after 64 hours, that it’s more likely to be diet related. Just a thought.
I enjoy hitting the hot tub, steam room, and sauna at the gym. While fasting I try to make sure my salts are good and that I’m hydrated. My body seems to enjoy it a bit more the longer the fast. Just my two cents.