I bought a pin prick finger blood test thing for blood sugars I think, and pricked myself when it happened and found I was within normal range, although I could have sworn that was the problem. I did find myself borderline low (3.5) when fasting once but still considered normal.
I've read several books about hypoglycemia and they usually point out that the rate your blood sugar drops during a hypoglycemic episode is more important than being near, or below, the low end of the range. And, your low level may be higher than the average low.
When I was certain that I was having hypoglycemic episodes I tested my blood sugar using a pin-prick tester and found that the meter could be off by a full point when using the same drop of blood for multiple samples. There is a quite a difference between 2.5 and 3.5.
Eventually I just learned to trust my symptoms and the remedy, not the meter. If my symptoms match those of a hypoglycemic episode, and eating quickly relieves my symptoms, then I'm pretty sure I was having low blood sugar and the food raised it. This is something that I have confirmed thousands of times by now.
I've also tested different foods to see how I feel after. If I eat an apple then I'll get a blood sugar high, with a blood sugar crash within 30-45 minutes. If I eat something that takes longer to digest then I can go for up to 2 hours before the hypo symptoms start again.
If I take apple cider vinegar before eating (often suggested to help increase stomach acid, and to slow the rate of blood sugar increase after eating) then I'll feel very tired. I think it slows the blood sugar increase too much in my case, so that my blood sugar takes a much longer time to rise to a normal level. This may be another cause of fatigue after eating for some people.