on the left side of my mouth one of my molars and the smaller tooth in front of it have brown at the base of the tooth near the gum line. I am not a smoker, got to the dentist faithfully (every 6 months), do not use a tooth whitener, am normal weight, eat extremely healthy and drink 8-10 oz bottled water a day. I do have an occasional glass of red wine in the evening. Why are my teeth turning brown?