Worlds are being reset

I had a creative superflat world in which I had a few things built. When I loaded the save and spawned into the world, the world was no longer a superflat, just a regular world. I though this was very odd, so I decided to exit and save. When I went back to the world, The world seed had changed to zero. Today, I loaded up my survival world, only to find that all of my items were gone, and I had spawned in a different location than I had saved in. Turns out I had a blank map in my inventory like the one you start with, and I had spawned in the default world spawn point. My world, however, was unaffected. All of my creations were still there. I decided to exit without saving, because I didn't want to lose all of my items.