I'm guessing oral thrush (Candidiasis), thus the Nystatin. You need to take that for a week or so for it to kick in though (doesn't go away in a day). Did you take the full course? Check with your doctor for other options since the Nystatin didn't work. Other than that, look up oral thrush on the net and see your options (I don't recall ever getting it).
My guesses. The yeast overgrowth is due to changes in the mouth/diet/immune system. So make sure you're taking your vitamin D, look into some oral probiotics (to help crowd out the thrush), maybe tea tree oil gargles, and see if there's specific things in your diet that seem to aggravate it.
Oh, and make sure you sterilize/replace your toothbrushes so you don't keep recontaminating yourself with that particular strain.