1.3a5 – something keeps modifying options siteurl

I’m not sure if I’m being hacked, or what, but something keeps changing my site url from https://mywebsite.net/ to https://mywebsite.net/cgi-system. This obviously breaks most of the links and the stylesheets. Any idea why? I’ve tried fgreping the wordpress php source, but nowhere can I see any mention of “cgi-system” within it.
Very, very weird.