As long as I can remeber as a kid I get this weak spell to where I get really weak and need to rest for awhile I have found that eating something helps alot, and goes away in about 10 min. I have checked my sugar while I was having one and it was normal. I am 31 now and getting more worried with it every time I get a weak spell. I have mentioned it to doctors but to some its just come back to see what happens. But I dont because of the lack of missing work or dont have alot of money to do any thing about.
The 2 things that come to mind are low blood pressure or low blood sugar. Since your blood sugar is normal when this happens, this is likely not the cause (although I do wonder what your blood sugar is like when this is NOT happening...maybe your blood sugar is usually elevated and when it gets normal, you feel the effects of a lower blood sugar more than someone who always runs normal. I have worked with several diabetics who run so high (>300) so often that a blood sugar of 150 makes them have hypoglycemic symptoms). Here is some information for you about hypotension and hypoglycemia. I do encourage you to follow up with your physician though since there may be even more to this (neurological, cardiac, pulmonary, etc.) and may not be related to blood sugar or blood pressure at all.
Best of luck,
Monica M. Skidmore, MS, RD