I had strange metallic taste on tip of tongue that lasted for a few seconds. frightened that it's a sign of brain tumor. obsessed over brain tumors.

Probably isn't...: See a doc and get examined. Likely will be reassured.

Obsession: No, not sign of brain tumor. More likely you tasted a drop of blood, especially if you are not flossing, or if your are overdue for a professional cleaning. You may want to consider seeing a Neurologist for examination to prove to you that you don't have a brain tumor, followed by a mental health professional to deal with your obsession.