accorrding to my doctor when you go along time without knowing your blood sugar is high, your body wil automatically adjust to that and you will feel normal even though its unhealthy. so when you blood sugar drops to what would be a good or "healthy" range to be in your body thinks it doesn't have enough sugar. I would definatly see a doctor asap. they will probablly put you on some kind of medicine such metformin and some kind of diet. it will take time but you need to start getting your numbers lower, cause when your sugar is that high for long periods of time, it starts sticking to your oragons and starts to eat at them. Thats what they just told me in the hospital. Hope can get some answers soon.
My mom is the same way. If she gets under 100 she starts feeling all shaky and sick. Her norm is just not everyoneelses norm. Not sure about how healthy this is, but its just the way she is. Id talk to your doc to see what he says about this.
I suggest you see your doctor and get a thorough health checkup. First thing in morning before eating, a level of 312 mg/dl is unhealthy.