I had the same thing happen to me, a very old filling stated *tasting* like that soon after I had a new filling done. The dentist re-did the old filling, telling me that there was a valent reaction between the old and the new causing the metal taste. Hope this helps.
hello there! i am going to school to become a dental assistant so i know a thing or two about this. my understanding is that they do not like to do a composite/white filling if the cavity is rather large because it can cause major sensitivity ... the material does not expand lke a normal tooth does to changes in temp. the amalgam filling has such a low amount of mercury in it that it will not hurt you. thats an old wives tale. if you haev any more Q's send em my way! hope this helps!