Please help! what is the cause when you get a metallic taste in your mouth (at a particular location)?

Taste: It may be dissimilar metals in the mouth touching one another or an old filling breaking down. See your dentist for evaluation.

Lots of reasons: Including, but not limited to: 2 dissimilar "filling" metals coming in contact leaking (failing) older restoration gum pocket medications gerd etc. Please see your dentist for diagnosis and resolution before problem gets out of hand.

Bleeding gingeva: A localized area of bleeding gingeva can also cause a metallic taste.

Infection/restoratio: It's either from localized infection that bleeds or from a defective metallic restoration like a filling or a crown..