I would have imagined that it may be geographic tongue but it probably would show up on a brush biopsy? Have you had a topical steroid prescribed to see if that'll make the white coating go away? If that's already been attempted, you may want to see if its a possibility of a side effect from medications that you may be taking. Another possibility may be an allergic reaction to something new that you may have been using such as mouth wash, toothpaste, makeup, etc.
Its probably not an STD. At first description, it sounds more like the above.