Insulin 'resistance' can be a difficult issue to deal with and understand as it can manifest in several different ways.
In your case, it may very well be a symptom of not producing enough insulin from the pancreas (due to beta cell dysfunction perhaps brought on by Nilotinib).
Or it can occur because your cells (skeletal muscle & fat cells) are no longer as sensitive to insulin and your beta cells are being pushed to produce more and more insulin. One way to know which mechanism is at work is to have an insulin test while fasting.
Normal weight people can also exhibit insulin resistance (pre-diabetes) when their fat & muscle cells are "full" and the body can no longer process excess sugar easily. In this case insulin would be very high (as it tries to shove the extra sugar into storage and can't easily).
Diabetes runs in my family (unfortunately) - and I have noticed when I put on weight (but still look thin), my fasting sugar rises considerably (over 100 mg/dl) into "pre-diabetes". The only way I can reverse it is to exercise, cut out carbs (by fasting) and lose the weight. I think of this as opening up my fat cells for future sugar input. Only then does my glucose level drop back into the 80's. But I have to be quite thin for this to occur. I'm close.
One inexpensive way to know for sure in your case is to fast overnight and in the morning before eating, test your glucose level and ketone level. If both are high, you have an issue.And as long as your fasting glucose is below 100 (below 90 is better), you are likely producing enough insulin.
Based on your observations, however, it is likely to be a nilotinib adverse event. You may have to switch TKI's. Fasting can be an effective strategy to help beta cells regenerate (https://www.masteringdiabetes.org/beta-cell-regeneration-diet/) assuming they have been damaged. Studies to show that when Nilotinib is stopped, beta cell function returns.