most of the times when we open big cavities because of decay and the bottom of the cavity is too near the nerve, it's common that some bacteria have already travel to the nerve and when your dentist clean that he exacerbate the bacterias making them to grow faster inside the nerve chamber... these provoke a normal response of the body to sepsis... the best thing to do is going to your Dr. and make him se the pathology... don't worry about the mercury and other metals, at these point...