I don't know if this will help you, since I don't use Code Igniter and don't know how the stuff is handled there.
But I usually use sub-arrays for everything I do in sessions, so no vars are in danger of being overwritten. So if I have a login-script it would only save to $_SESSION['login'] (for example $_SESSION['login']['password_hash'])
If that's not possible with code igniter or that library always deletes the session (what kind of library would do something like that???) you have to save the data somewhere else. Either in a hidden form field in the login-form (don't forget to serialize the data first) or in a Cookie.