I suggest going to see your regular primary care doctor
or your cancer doctor
. The first question I would want to know is whether or not you actually measured your blood sugar level and found it to be low. Many people have intermittent symptoms of fatigue, which are commonly attributed to "low blood sugar." However, in the absence of having diabetes or of taking diabetes medications, objective low blood sugar is actually very very uncommon. There are potential causes of objective low blood sugar in people who are not diabetic, such a low levels of cortisol in the blood stream, or tumors that produce insulin, but these are not common.
I think this is unlikely related to your prior history of cancer, although there are a few caveats, since I do not know your cancer treatment history or what type of cancer you had. For example, as I mention above, a very rare type of cancer that produces insulin can cause low blood sugar, and if this recurred than a drop in blood sugar could be a sign. Also, some cancers can invade the adrenal glands, which could lead to low cortisol levels and drop the blood sugar; again this is a rare event.
I suggest going to see your regular primary care doctor or your cancer doctor. They can investigate your symptoms and help you figure out what is going on.