Google on "Google Panda", it has been released last couple of days on ALL website with English language. Not limited to the US but internationally. Could be the cause...
I would say don't panic, I saw a big drop on April 6th on a bunch of sites but it bounced back to normal levels in a few days.
It could be a victim of the Google Panda update, your site might be on sites with duplicate content and you might be put in the same brush, either that or give it some time and things might revert back to normal