Get a copy of the results of all your tests, especially the blood tests. Most of the glucose tolerence tests are to rule out high readings. They seem to think it's fine if they are lower than normal. I had low blood sugars for years which caused a lot of dizziness, etc. It seemed that if I drank a glass of orange juice, within an hour I could barely keep myself awake. That's why diabetics drink OJ if their sugar drops. As more information became available I realized that snacking on anything sweet or starchy would lead to the dizzy and sleep. I also had low blood pressure, the longer I would stand the lower it would seem to go. I had a terrible time trying to get through a grocery store. I had to change my diet and the way I ate my meals. The protein and greens need to come before the starches. No sugars for a while. NO artificial sweeteners. For a while I was carrying around celery sticks to munch on. I sometimes had carrot sticks, but sometimes even they had too much sugar. When starches and sweets come first the body tends to send out lots of insulin to take care of it. Then the blood sugard drop as the insulin comes on.
I also had a tilt table test done which did show some slight problems. My primary doc gave me proamatine a few weeks later. I guess he had been doing some reading and came across that medication. It was a really big help for me.
I do hope you are able to find an answer soon as I know all too well what you must be going through.