As I understand it

I didn't realize that I was hypoglycemic until I was well in my forties. I ended up in a hospital in Dusseldorf after having minor convulsions on a plane from Milan.  We'd eaten lots the night before the morning of the trip, we were late for the plane and had no time for breakfast - but just time for a quick expresso.
An American physician in the hospital explained that I'd dealt myself the perfect hand for hypoglycemic shock, and with his and a number of doctors over the years, I discovered what happened then and what combination of events creates a hypoglycemic reaction.
In very simple terms - here's how sugar travels through your body.  When we drink, say a glass of apple juice, the pancreas excretes insulin to aid in metabolizing the sugar.  In a hypoglycemic, excessive insulin is excreted and rather than slowly metabolizing the sugar, it quickly lowers the amount of the sugar in the blood - in some instances far below normal levels.  This process is more severe the faster the sugar is ingested - so apple juice goes down very fast, apple sauce not so fast and an apple (depending on how slowly one can eat it) not fast at all - producing equivalent reactions.
Usually, the headaches or emotional roller coaster reactions occur at this stage and it is uncommon that it goes further. Under certain circumstances, however, the reaction can be far more severe. If one is involved in strenuous exercise, a sudden shock (like fright or diving into cold water) or drinking caffein (more on caffein later) sugar is even more quickly metabolized and a second level reaction occurs.
If the sugar levels in the body drop low enough or fast enough, the adrenal system will react in emergency mode, just as if there was something to fear or need for immediate physical action.  When adrenaline is secreted it quickly metabolizes what sugar is left in the system - to which the pancreas responds with more insulin, depleting blood sugar further.  At this point the reaction is usually an incredible headache, emotional fragility and sometimes, nausea and vomiting.