I'm not diabetic but I get those same symptoms a few hours afterwards if I eat a lunch that has no protein (such as a fruit salad or bagel & banana). I am sure it is a drop in blood sugar, and it is alleviated by eating or drinking something. I do not get it if I eat a chicken salad, or sandwich with meat/cheese for lunch. I think it is a form of hypoglycemia and I am just careful to not eat a high carb/no protein lunch.
Try to track exactly when it happens and what, in the hours before, prededed it.