Why do I have a metal taste in my mouth?

Multiple causes. A metallic taste in your mouth can be indicative of blood, certain antibiotics, reflux, or a number of other things. Associated symptoms would make a big difference on the next steps in evaluation and treatment. The bottom line, go see your doctor for evaluation, especially if this is persisting.
Multiple causes. Its difficult to give a great answer with limited data. Many things can cause metallic tastes. Medications, foods, hormonal changes all can alter taste perception. High metal levels in foods and water can cause a metallic taste. It is possible that it could be from a metal of a crown or washout of a crowns cement. Start with a physician to start ruling out possible systemic causes.