Google has rolled out Google Panda 4.0 update yesterday. Google’s Panda Update is a filter introduced in February 2011 meant to stop sites with poor quality content from working their way into Google’s top search results. Panda is updated from time-to-time. This is a new Panda algorithm, not a refresh that we’ve seen almost monthly. Please refer to the below article for more information.
http://www.seroundtable.com/google-panda-4-18583.html