I believe what you are referring to is hypoglycemia.
http://www.mayoclinic.com/health/hypoglycemia/DS00198
I had the exact same problems as you, so I got tested for diabetes. This was an overnight fasting test. In the morning, I had to wait at the lab for 6 hours. They started by giving me a glucose drink and then they took blood samples every half hour. Yeah, I ended up looking like a drug addict with all the pin holes in my arms
Turns out I do not have diabetes (thank god) but I produce higher than average insulin when I take in a lot of sugar. So my body kinda overcompensates, and after 3 hours, I'm left with low blood sugar, which causes me to crash.
Doc advised me to eat less but more often, say every 2.5 hours, and to control my sugar intake. I've been doing that and it definitely feels better.
You should get tested for diabetes just to rule it out.