I get a metallic taste in my mouth every once in a while. Why is this?

May be from fillings. Different metals are often used when teeth are restored (fillings/crowns). Saliva can create a connection between these different metals causing an electrolytic activity that creates the metallic taste. Other reasons can be diet-related or from nasal/sinus infections.
Could be.. A fungal infection called candidiasis. See a dentist or oral medicine specialist for help.
Metallic taste from. Some of the more common causes of metallic taste are poor oral hygiene, deteriorating silver fillings, some presciption medications and otc vitamin supplements. See your dentist who might be able to help you figure out what is causing it in your particular case.
Many things. Metallic taste is associated with galvanic reactions in your mouth, foods you eat, parotid gland cancer, a visit ti the dentist is a good idea.