if you haven't already done so, run an immediate virus/adware/malware check on your computer. it may have been compromised by something downloaded invisibly from another website.
if you see something like this, report it immediately to Admin.
also report it to every moderator who appears to be online at the time.
neither Admin nor the moderators as individuals are online 24/7, but there's usually at least one available at any given time.
i've sent Admin the link to this topic so he can do some research. he might be able to figure out what happened. it's not likely that the problem originated here, but he'll certainly want to verify that beyond the shadow of a doubt.