As per Matt Cutts blog post, Google started rolling out the newest version of the Penguin webspam algorithms on May 22, 2013.
Only 2.3% of US queries will be affected and the goal of the recent update is to more impact the webspammed sites.
During the roll out of Penguin 2.0 some websites have been downgraded as a result of the negative SEO which perhaps is an indicator that the negative SEO can be now easier used by your competition to remove your site from high ranking.
If your websites has lost the ranking after May 22nd then perhaps you need to research whether your site have been targeted by the negative SEO.