It is most likely the flight deck called the Purser or one of the FAs before making the P.A. announcement for you to hear.

If you're on a 744, you can *sometimes* hear the intercom calls doing this on Channel 9 (listen for the "phone ringing").

Do you know what altitude you were at when this happened?

If the FA asks you to prepare for landing, you should follow their instructions. You are only asking for further hassles if you refuse to oblige.

As much as we may enjoy knowing as much as we can about the operation of the flight we are on, we as pax are not always privy to the relevant information (e.g. that call from the cockpit to the FAs). Also note that the captain's PA announcement is directed at the FAs, which happens to be overheard by pax. Just another way of communicating that often helps the process run more efficiently.