Brown Tongue and funny taste in mouth

Hope someone can help me here, because i haven’t a clue what to do. About two and half week ago , i woke up with a brown tongue. At first i thought it was because i was eating fishermans friends or a curry that night.
I thoroughly cleaned my tongue but the brown stain remained, then i got a sour taste in my mouth, even my teeth felt and tasted sour.
My doc says could be B12 deficient-nothing done.
Chemist gave me Dactorin-fungal infection.
Dentist advised Corydyl mouth wash.
Still brown tongue,and the taste has gone from sour to a buttery taste.
Never has this happened before can anyone relate to this,or offer advice. please.