Were your sites impacted by Panda, Penguin or another zoo animal? Then you may want to check out the 53 changes that Google made to search in April. http://insidesearch.blogspot.com/2012/05/search-quality-highlights-53-changes.html The blog post located at the URL above is from Matt Cutts.
It's as official as the oxygen you and I breathe. You can go to http://www.google.com/insidesearch/ and click in the "blog" link in the upper navigation bar and be directed to the homepage of the blog post I initially posted.
Thanks for sharing it, I need to read it properly so we can understand what will be the future with Google..