Weird Generations of Blocks

I started a world in survival and started exploring after mining for a few hours. while exploring I saw a tower of sand. I turned around to then find a weird build of just floating sand and dirt. I then crashed and logged back on and the build changed. I even have image proof and there was nobody else on the world either. I also know that I have no viruses or somebody else doing this. This has happened across two different worlds and also happened when I was playing with a friend. However, when playing with my friend I could only see it and not her. I am now scared to play because I don't know what to do. The first picture is before the crash and the second picture is after the crash.