Hi Rufus. This can sometimes be caused by diabetes...can be a thrush infection (a yeast) than v often develops when sugars have been a bit high...you need to see your GP. Don't try and treat on your own..I know as well this can be extremely painful. Hope you get it sorted @Rufus3013
When you've got it sorted, invest in a tongue scraper (sounds gross, I know) or brush off your tounge with a toothbrush but gently or you'll just make it sore. I find it helps to keep any gunk from settling there. Certain foods also tend to give my tongue a coating - nuts, certain types of curry so I think it must be the mix of spices