Where did I end up in Minecraft?

Where did I end up in Minecraft?

Under the world, in the "Void"

Since you have chosen the world "flat" (I guess that's what it's called), that's so-called the end stop at the bottom gives nothing more

Is like that in every world

So under the map in the void.

You have built yourself down so far that you are now under the world in the void. Fly up to sleep again then you are back in the normal world.

You are under the world.

That's called Void.

Just fly up and you're back in the normal world.