A brief sudden change in a normally constant quantity: a pulse of current; a pulse of radiation. b. Any of a series of intermittent occurrences characterized by a brief sudden change in a quantity.
Some pregnant women experience a metallic taste in their mouth during the early stages of pregnancy, because of the hormone changes. It's also fairly common to crave certain foods, or go off certain foods, and have a heightened sense of smell. Learn more about the signs and symptoms of pregnancy.
Ingestion of pine nuts is a recognised cause of a metallic or bitter taste in the mouth that may occur one to three days after eating the nuts. Not everyone gets the problem and why it happens is unknown, but quite a lot of research has been done on the topic (see ncbi.nlm.nih.gov/pubmed/21559093).