Achievements turned off on their own.

I have been playing on one survival world for a while now. I have been receiving achievements up until yesterday. I noticed that in my setting is says that achievements are unable to be earned in this world. But I am certain I didn't turn on cheats or creative mode, because I wanted the achievements. It also asks if you are sure you want to change your settings, because you won't receive achievements. I'm positive I never would do anything to turn off the achievements.