Check out food allergies - many don't show up in the normal tests but Dr. Coca, a renowned immunologist, developed the Pulse Test to determine what foods are causing you problems. If your pulse is more than 7 beats faster after eating than it was before you ate, you have a food allergy to something you ate. It can take a long time and a lot of effort to pin down the source as some people find they are allergic to most every thing, but it can be done. Here's the book by Dr. Coca on the pulse test:
http://www.soilandhealth.org/02/0201hyglibcat/020108.coca.pdf
Here is a link to "Brain Fog" which includes a reference to the pulse test and also has a load of great information besides food allergies.
http://books.google.com/books?id=ukLtoxo0zFwC&pg=PA114&lpg=PA114&...
[11:11] blood glucose 84 [11:21] blood glucose 95 [11:31] blood glucose 106 [11:41] blood glucose 96 [11:51] blood glucose 103 [12:00] blood glucose 115 [12:10] blood glucose 112 [12:21] blood glucose 118 [12:30] blood glucose 101 [12:40] blood glucose 103 [12:50] blood glucose 108 [13:00] blood glucose 104 [13:10] blood glucose 105 [13:21] blood glucose 99The 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).