Right now there is a global Wordpress brute force attack taking place where up to 90,000 individual IP addresses have been detected as involved.
http://blog.sucuri.net/2013/04/mass-...r-reality.html
http://blog.sucuri.net/2013/04/prote...e-attacks.html
Check your server logs, ensure you have strong passwords and preferably don't use "admin" as your login name.