Most of might know one recent update from Google i.e Penguin Update. This update was made on 24th April. The update targeted web spam, website that were involve in malicious link exchange or link building, websites that were following black hat SEO techniques to improve rankings in search engine. But before that Google also made one update on 19th April That was panda update and targeted overly optimized sites. If your website is hit recently then check your traffic on and before this date. It will help you identify update which hit your website. Hope this helps.
Buckle in. Google is on a rampage. And search results seems to be pretty much all over the place right now.