It's too bad they just didn't give you a glass of juice and let you continue the fast. My guess is the juice would have fixed the problem. According to the fasting experts the blood Sugar is supposed to stay normal during a fast so I would be interested if anyone can come up with an educated answer to your question.

I will say that I was reading a fasting book a long time ago. In that book something happened to a guy on a lengthy water fast. The doctor just gave him some juice and continued him on the extended Water Fast for several more days. The question is what happened to cause the doctor to give him juice. I don't remember for sure. I tried to find the literature to reread it so I know for sure, but could not find it. According to my memory, which may not be serving me right, it was a blood Sugar issue. I posted this a long time ago and was told that would be impossible and my information is wrong.

I really would like to know if a Water Fasting person can have a blood Sugar issue. I hope someone in the know answers you.

My guess is that the liver failed to produce some more glucose, through gluconeogenesis -- a process that needs to happen at any point of time when the body had burnt all availabilitites, including muscles, blodd glucose and liver storage, and does not get glucose/fructose/lactose/ citric acid /pyruvate/malate from outside. All the enumerated are getting into a cycle called Krebb's cycle and under normal circumstances burn or generate glucose. So, when none of the above are consumed, gluconeogenesis needs to occur through transformation of free amino-acids (fully broken down/digested protein) into glucose.
It is plausible that you need to replenish your body glucose before starting a new fast.
These are speculations, I don't think the supervised fasting Science had it all elucidated.