Here's the full story. January 2016 was the last time I visited (I know, it's been a long time, but I'm going back this month). Back then, she noticed that the back of my 4 top front teeth were being chipped away because of my lower teeth (probably some mild grinding during the night), and she prescribed a bite guard, which I've been using since then. After that visit, I took a picture of my back teeth - BEFORE - https://www.dropbox.com/s/kyadaigydsmxvhc/
This month I noticed that these 4 teeth were much more stained than usually (sorry for blurriness, it's not easy to photograph your back teeth, it helps if you zoom out) [image link removed]/drive/folders/0B8eNDSu_E9YeVFVscT
I get the impression they are more chipped away than they were before, and they are also showing a brownish / almost yellowish color. Have I destroyed the first layer of these teeth? What could this be?