I start writing this post because i have a strange page

I start writing this post because i have a strange page on my analytics. This page is not on my site.
On choosing witch site has the problem (writing that post) i found again some sites that do not belong to me.
I have clean that less than a month ago and change my password to wpmu.
It seems that i have been hack and the problem is keeping going on.
Any ideas what i can do?