I'm not saying that it's low blood sugar, but that is something that can sneak up on you.
Headaches are one of the first signs, your brain needs a lot of sugar.
Eating often (5-6 times a day) and small doses of protein (cheese, etc) may help.
It is seemingly random, even with a good breakfast, you can get hit with it in the morning anyway.
I ran into low blood sugar before taking HRT.
I've had a few nasty espisodes (almost falling down) before I learned how to recognize it.