Blocks won't save

so I had started building a new house and a couple of days ago I noticed that when I had logged back into the single player world the new blocks that I had placed did not save. I thought maybe it was just a glitch and re placed the blocks on the inside of the house. However today I logged back in to continue with the building and I once again found that the blocks that I had placed inside of the house had not saved again.

so I did a little experiment.

I went into a few older worlds I had and placed some blocks. I then saved the worlds and quit the game. I then go back into the worlds and none of them had saved any of the blocks that I had placed! 

This is very annoying and makes the game unplayable.