I do not think you dug up nitrite. It could be something rotting in the substrate. You dug it up, and it produced more ammonia than usual. The system had more bacteria to feed on ammonia than the bacteria to feed on nitrite. So the ammonia was gone rather quickly, but the nitrite needed extra time to go down to 0ppm again.
Overfeeding not only can result in direct ammonia pollution from the leftover fish food. Even when all the food was eaten, the fish will produce extra waste and in return it means more ammonia anyway. When I feed my fish, I make sure there is no leftover at all. All fish food is gone within half a minute or so. Any more than that would be too much.
