What are you feeding your cat? If the food your providing is high in carbohydrates, given when you're testing in relationship to the possibility of a high carb food, the food is likely influencing your cat's blood glucose level. It can take more than an hour for the influence of food to not have an effect on the numbers. The symptoms of diabetes typically include a high water consumption and a greater amount of urination. Also, diabetic cats typically have a ravenous appetite.
You may want to schedule a vet visit. The vet can run a fructosamine test which will give you an answer to whether your cat is diabetic. I would not suggest giving your cat insulin unless you have a reliable diagnosis. There can be a number of concerns that could potentially raise blood glucose levels.