Feed hacked

I have a big problem with my Feedburner
feed showing spam ads while my site’s feed shows my site’s content.
When I troubleshootize to check my site’s feed validity, everything is
reported to be ok. When I do the same to check the Feedburner feed, I
get errors that it’s an HTML page. Resyncing doesn’t work because
then it reports a problem with my feed. Since the validator does not,
I can’t figure out what the problem is.

Basically, Feedburner sees a different feed than the validator and I
do. I thought it may be an .htaccess to detect the Feedburner bot and
return a different feed, but it’s not. I’ve updated every file to the
newest version of WordPress and disabled all plugins. Still no luck.