That's an electrolytic reaction caused by two or more metals. Your fillings alone can cause an unpleasant tingling sensation or nasty metallic taste. In each case the sensation is caused by an electrolytic reaction similar to what happens when you put certain bits in a horse's mouth. Copper mouthpieces have long been used to stimulate salivation in equines. Copper is an electro-reactive metal with a low oxidation rate, as evidenced by its discoloration over time.
German silver is an alloy of copper, nickel, and zinc.
Proponents of these expensive bits claim they are superior because they contain no nickel. Sweet iron is likely not pure iron, but a mixture of iron and carbon combined to create some form of a carbon steel. Bit makers claim that it tastes sweet to the animal and therefore keeps the animal's mouth moist. In my own experience sweet iron does not seem to cause extreme amounts of salivation, but this no doubt varies from equine to equine. Whether sweet iron tastes good or bad, sweet or bitter, is academic.