Visits & Unique Visitors stats lower then normal

Hi Piwik Team

The past week i have seen Piwik display stats for visits & unique visitors much lower then normal for my site. My site normally has approximately 10,000 visits and 6000 unique visitors (Mon - Fri) and half that Saturday and Sunday. The past week both Visits Over Time & Visits Overview are reporting under 1800 visits and 1000 unique visitors.

Has anybody ever had this experience with their Piwik instance and know why this is happening ?