Hmmm.... so, SMF is the only thing running on your site?
Did you give Sir Osis of Liver the FTP access that he requested?
If so, he can hopefully check your files and logs.
I would recommend also checking with your host and asking them to do a filecheck of your site.
SOMETHING has been compromised... we know of no security issues in 2.0.13, so either the used a different route (another software, another site on a poorly configured server, etc) or there is something that we need to track down with the help of you and your host.