I have absolutely no idea what it could be, but I thought I'd tell you what happened to me once in case it's similar to you...
I had a major thyroid/endocrine crash and started gaining 21 kilos over 9 months. While the weight was piling on (despite me eating all raw, and very little, and exercising like a maniac), I went to a naturopath and came home with a box of supplements which I started taking. After a week or so my teeth suddenly got a horizon line of brown on the bottom, white on the top. All my teeth had it and it was so noticable, it looked like they were rotten or something, but they weren't!
It took about three weeks, but the horizon line kept lowering 'til all my teeth were whiter than before. No one has ever been able to tell me why this happened, nor has anyone I know ever heard of this in someone else, but it obviously had something to do with the supplements I was taking.
The only ones I can remember of the many he gave me were, barely grass, liquid zinc, a protein powder, Vit C I guess.... That's all I can remember but there were more.
Have you done anything different lately as far as supplements or diet are concerned??