It's a long shot, but I read this in the comments on TWD's FB page....
Someone said they think carol found lori's body, took the placenta and shit out (hence her knife being found), and took lori to safety. Reason this doesn't sound COMPLETELY crazy: no remains of Lori were found. There's always a body/skeleton. That walker looked full but he didn't eat the bones and shit.
AND the blood trail suddenly stops like Lori was picked up which brings us back to the carol possibility. And the phone call is carol from another part of the prison.
(this is all some dude's guess on their FB page)
Like I said, a long shot, but hmmmmm..