I have this exact thing now!! Has felt like this since just over a week, when I went to Sparty, (salt thermal pools with chlorine added) and I went to the doctor who had no idea and said nothing looked abnormal. She merely suggested to avoid eating/drinking stuff that is too hot to irritate but no idea of cause. Any progress on yours? Mine almost feels like I haven't brushed my teeth in days and all the fluff /build up you normalls get on your teeth after a night out is sitting under my tongue, but it feels like the straight after I've brushed my teeth....
Had essentially the same exact symptoms described by the original poster: Basically, a slightly rough, almost "sandy" feeling, felt when probing with the tip of my tongue, on the floor of my mouth under the tongue. It came on suddenly and seemed to spread just a little bit, over the next few days, much as described by some of the other posters to this thread. There was no pain, and I really couldn't see anything unusual when I checked in the mirror. Long story short, after about 5 or 6 days it went away completely and hasn't returned, which I suspect will be the case with most of those similarly "afflicted." My thought was it may have been related to what I was eating at the time, but I've eaten the exact same thing since and have had no problems.
One week later I started with this grainy feeling under my tongue & inside of my gums. I am going to my Docs, as I wondered if it might be yeast related?