Is this some metabolic disorder?

In the past year I've been reacting weirdly to food. I can't take too many carbs in for breakfast, e.g. cereals are a no-no. If I eat any, I can go to bed immediately. I get totally tired all of a sudden and basically fall off my feet into bed with chills.

So I suspected high blood sugar and diabetes, got tested, nothing. My glucose is 90-95 (several occasions), while the diabetes threshold is 120.

So I took to eating fruit in the morning and then discovered I can't eat large (normal) quantities over the day either. I have to eat little and often.

I thought it was all the carbs. But today, for the first time in months, I had breakfast made of 1 poached egg, 2 strips of bacon and 1 slice of bread.

Immediately afterward, I got extremely tired and had to lie down in bed with chills.

I don't get it?

What am I doing wrong? The doc says it's all good.