Blackish brown tongue?

ive been having stomach pains, sometimes its a blunt pain where its all over sometimes its a sharp pain.. but it normally comes along with nausea.i went to the doctors and they thought it was a gall bladder problem but its not.. its been going on for about two and a half weeks now.. and this morning i woke up and i looked at my tongue (for some odd reason) and its like.. coated with a blackish brown color?.. any ideas?