Google has confirmed they have pushed out a refresh on the Panda algorithm on Friday, April 27th. Google didn't gave any other information about the update. Last week has kept us quite busy with constant updates from Google with Panda 3.5 update on 19th April, then Penguin update on 24th April and now panda 3.6 update.
Just follow white hat SEO techniques and you'll be fine. No matter how fast Google updates, you'll always be safe