A change of LUD without e-mail or any change in case status message is pretty common. Many people get several LUDs before their case is approved, some get none. Sometimes such LUD means something (approval, transfer, RFE, fingerprint info receipt, etc.) and sometimes it's just a routine interoffice cross-reference procedure. I tend to view a LUD as a positive sign - it's better to know that your case has been touched for whatever reason rather than collecting dust on some shelf.
