Your pancreas is trying to keep up. It produces insulin to offset the excess glucose.

I don't know enough about the pathways the chemo is effecting to fully understand how it is changing things.

In general carbohydrates cause the pancreas to put out insulin. Protein only causes a slight insulin response, although if eaten along with carbohydrates will be an additive response and more than if protein along or with fat. Fat generally is considered to have no insulin response, although again if eaten with carbohydrates it will cause the energy to use different pathways because of the insulin response by the carbohydrates.

The best way to lower blood glucose is to not put the glucose chains in the mouth to begin with. Carbohydrates are just long chains of glucose that get broken down into glucose upon digestion. "Fast" carbs like things made out of flour or sugar get broken down very quickly. More complex carbohydrates get broken down more slowly. Avoid the fast carbs if you don't want glucose spikes.

The other way for glucose to rise in the blood is through gluconeogenesis by the liver. If a person is eating zero carbohydrates (say on a meat carnivore diet for instance) there are no carbohydrates being digested to put glucose in the blood. The liver will put out all the glucose a person needs. Therein can be a problem if a person is insulin resistant and has metabolic syndrome (early stages of diabetes or pre diabetes). A fatty liver will also add to this problem. So a person can eat zero sugar (carbohydrates are just undigested simple glucose) and still have blood sugar levels above what a healthy person can have. The liver should never put out excessively high amounts of glucose. But a damaged liver along with even modest amounts of carbohydrates can elevate blood glucose to the levels you are talking about.

You might check out a very low carbohydrate (under 20 total grams of carbohydrates a day) diet. Or even a carnivore diet. For sure you should look at cutting out all "fast" carbohydrates like potatoes, bread, pasta, anything with sugar in the label, rice, etc. When you cut out carbs for energy the body has to get energy from somewhere. The other source of energy is fat. So a low carbohydrate high fat diet would be one way to replace the lost energy. The body starts to burn fat for energy instead of glucose. A ketogenic diet as ketones are produced from the fat. If you don't eat enough fat the body can use protein as an energy source but it is very inefficient. Protein is generally a building block, not an energy source. So if you cut out the carbs and only eat lean cuts of meat the body will be starved of energy and be forced to use protein. 

It takes a week or three or more to transition into fat instead of glucose burning mode. There are lots of on line doctors that have good videos of how to transition into a low carb or ketogenic style of diet. It is more of a lifestyle than a diet.

I don't know how your chemo is affecting your system. But I know I went from injecting myself with 100 units of insulin total in four shots a day to zero insulin in two weeks after transitioning into a low carb diet lifestyle. With better blood sugar control than I had while on insulin shots. Have not used insulin for over 4 years now and diet alone controls my blood sugar. Also got off diabetic medicines and blood pressure medicines by going low carb eating.

I'm not a doctor so don't do anything because it worked for me. Do your own research and decide for yourself. I'll give a few links to doctors I think have good messages on the subject.

https://www.youtube.com/results?search_query=dr+ken+berry+reversing+diabetes

https://www.youtube.com/results?search_query=dr+david+unwin+reversing+diabetes

https://www.youtube.com/results?search_query=dr+ken+berry+keto+101