i think you are being hacked. open options, and hit force update. isn't that simple?
Save files are saved on to the computer itself. There is no chance someone made a world, and hacked his computer, then somehow accessed his files. The only possible way is if someone other then you had access to the computer and your Minecraft/Mojang account, or if you just have a really bad memory
EDIT: They wouldn't need access to your account, just the computer. This is due to the fact that they just drag and drop the world. They could still have gone into your account and made the world in-game.
1
1
EDIT: They wouldn't need access to your account, just the computer. This is due to the fact that they just drag and drop the world. They could still have gone into your account and made the world in-game.