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Hypoglycemia tends to fall into two types - fasting hypoglycemia, and reactive hypoglycemia. With reactive, blood sugar will fall low about 2-3 hours after eating a meal rich in sugar/carbohydrate. The 3 day fasting test should be negative for people with this type. But with fasting, blood sugar falls on fasting. This is IMO a lot more serious, although less common, and needs to be diagnosed asap.
Various conditions can cause either form, so the cause needs to be investigated.
I seem to get similar symptoms, especially fatigue and brain fog for days at a time. For me, this is worse than low blood sugar spells. I think it can be caused by the hypoglycemia, which was my own question on this board "diagnosed hypoglycemia". I'm not sure so maybe someone else can confirm that...
There's tones of information on the internet about symptoms and the difference in types of hypoglycemia, do a little research. But bare in mind there's some conflicting information about so don't take it all for fact - and keep at those doctors