There are a number of things that can impact on your blood glucose other than food, including exercise and stress. But you may also have unknowingly eaten carbs.
One of the most common causes of a surprise spike is hidden carbohydrates - sauces, condiments and coatings can all be loaded with sugars, particularly if you’re eating out or having processed foods.
Exercise can raise your blood glucose as your body releases energy in response to increased demand, particularly if you are doing a high-intensity activity.
Stress can also cause a spike particularly from something like a near miss while driving, but also physical stress such as a cold, or extremely hot shower.