You may be experiencing a Google dance. Are you sure your positions changes are confirmed (the result may differ if you are logged into your gmail).
A Google dance is the term used to define a ranking fluctuations in SERPs over several days (can be longer) variations in ranking can be huge.
Three main reason might be the cause of a Google dance.
1. You deeply changed you page content or Keyword optimisation an Google is trying to find it's right spot again
2. Google is reindexing this section of the web
3. Your site just started ranking or is young and google is trying to find it's good position.