[PC] game difficulty has been constantly changed from nightmare to casual

More details, please.

Does this happen while you're directly engaged with your computer and you set the Difficulty, play for a while, quit to do something else, then re-start the game only to find the Difficulty has changed?  Or does it only happen after you've been away from your computer for a long while?  (In other words, could someone else in your household be playing as well?)