Google has just release a new project named Caffeine. It will provide a new architecture for its search engine, improve robot crawling and results relevancy.
This evolution means that website's rankings will change. Your positions on search results may increase or decrease in few weeks, according to your search engine optimization.
To avoid fear, Google allow us to check our future rankings on this page: www2.sandbox.google.com and you can send feedback about results differences via the link "Dissatisfied? Help us improve" at the end of the result page.