Hey Annie,
I also suffer from something very similiar to this. I do not have hypoglycemia or diabetes and have been tested for these. I have a normal fasting blood glucose.
I have always had issues with temperature balance (since childhood). Mainly from heat. If I get over heated, I tend to pass out or black out (where my vision goes black). It will usually happen very quickly and not necessarily because of the weather outside. I will break out in a cold sweat, be trembling and become extremely lethargic. It is very odd and the only thing doctors could ever tell me about it is that it may be something to do with my thyroid or my migraines. I have had countless tests, and nothing seems to be out of whack. So I just keep myself indoors, don't over exert myself and try to eat balanced meals every few hours.
I find eating high sugary/carb foods makes it worse. Extreme changes in temperature make it worse as well. I live in South Florida and the weather here is very unforgiving. Especially now. It is only 85 and I get a bit of lightheadedness just walking to the mailbox.
I'm sure there is some type of explaination for this, but I haven't been able to find it. I hope you are able to find some answers.