I can't speak to the yellow sore, obviously this needs to be explored carefully in the immediate future. But when suddenly you develop a white tongue, throat etc, it is usually Candida, a fungus infection that can be cured in a few days with the use of an anti fungal drug. One simple test for Candida is that it can be scraped off the surface of the tissue in most cases. This removal dosen't cure it, but it may ease your mind that it is something simple like a fungus bloom. There are some pictures of Candida in the dental section of the main site, under complications.