Minecraft Does not save my single player worlds

it started in 1.3.2 i loaded up my world and built my house. When i got done building my house I had to eat lunch. Later when i came back to my world the house was gone. So I built another house thinking that I didn't save properly. Then I quit my world. I came back. The house was gone again.

1. build stuff in a world
2. leave the world
3.comeback
4. its not there