Many call this hypoglycemia (low blood sugar). But it is a natural healthy thing that happens when you go too long without eating. I had it but cured it. I cured it by not eating for days. It is called fasting. These are actually cleansing symptoms like if you stop smoking or stop taking heroin.
Look at this page on
fasting. Then search the page (press and hold control while pressing the "F" key) for "hypoglycemia." It gives an actual story about this and how to tell if this is actually the case or if it is something else like a brain tumor, ebola, hyper-gamma globulin anemia or Munchhausen syndrome by proxy.