75g of every 100g of Special K is carbohydrate so its not very surprising your blood sugar reached 16.4. If you had the manufactuers typical quoted 30g helping (most would have more I suspect) with 125ml milk you would be consuming 28g of carbohydrate not counting the sugar sprinkled on top.
Why not give something else a try such as egg on toast, unsweetened yoghurt & nuts, glass of milk. That''ll fill you up until lunch and let us know what the bs reading is this time.