If you have an infection or virus, that too can influence your
blood sugar levels. Steriod medications can also influence your
sugar readings.
It may be useful for you to keep a food diary. Do bear in mind portion sizes - you can find information on the web regarding portion sizes of different foods. Do remember that all food is broken down by the body into energy. All refined carbohydrate foods (bread, cakes, biscuits, lemonades) will spike up your sugar levels very quickly.