I’d basically second what others have already said, and just add that something to be aware of in general regarding foreseeable difficulties fasting is ‘reactive hypoglycaemia’. It’s not a disease or anything, just a description for the way the body reacts to a sudden lack of sugar after it’s just dumped a whole bunch of insulin into the blood to deal with the sugars that were previously coming through. The body doesn’t like to have a whole bunch of insulin sitting in the blood with no sugar to deal with, so it freaks out a bit to ‘persuade’ you to ‘Just eat that apple, please! Think about those sweet, sweet sugars!’
Like another poster said, it’s a totally different experience to fast from an otherwise keto — protein based — diet, versus fasting from a regular high-carb western diet. At least that’s my experience and that of many others; it’s much easier to go without food when the most recent food eaten didn’t trigger the body to go crazy if receiving no follow up. I’m not suggesting you go keto, just noting this is a noticed benefit for those who do.
This may or may not be related to the specific issue you’re having though (that’s between you and your doctor).
And personally even though I’m suggesting you be aware of it as a general phenomenon (so maybe take notes of your usual diet before going in to see your doctor/nutritionist, in case they spot something helpful?), I doubt your issue is reducible to this. I don’t know why overheating would be associated with reactive hypoglycaemia. So definitely talk to your doctor, for sure. This doesn’t sound ‘normal’, to me.