Depending on your lunch at 2hrs your levels were probably on the way down. If you had done nothing for the next 30min they would have more than likely been lower than the 12.3. your exercise will have increased your need for energy and used up even more of that excess. So some of that 3.9 will have been used up anyway but quite a bit of it was used fueling your walk.
For me it would be more important to also know my level just before my lunch, then I would know if it was my meal that caused that 12.3