If you haven't, you might check your own blood sugar levels with greater frequency than you get with a lab test. For example, using my own monitor I watched my levels immediately after a meal:
The high frequency of testing allows me a clearer view than a single sample. Of course it means a lot of pricked fingers.
The other thing it could be is dehydration. Splanchnic pooling (blood going to the digestive tract) and water going into the intestinal lumen to help with digestion can take away blood volume from your brain. Other signs to look for include histamine release (red eyes, runny nose, sneezing) and posture-change hypotension (dizziness after rising to stand or to get up from lying down).