Cordless guitar fabric

I got something like a "hole" in my tongue, kinda like an ulcer, what could be the cause? - picture of an ulcer on gum

Here's a picture of my tongue lol

http://i59.photobucket.com/albums/g281/s ...

As you can see, there is this kind of ulcer on the tongue, brushing teeth everyday and everything, but when I wake my tongue, as it is ended, yellow. Perhaps it is has nothing to do with it, but I was really thristy too well, and when I woke up after drinking water, the kind of disappeared, it could be dehydration? What could be causing this?