Since you were apparently amending your existing reservation from 8 to 9 nights then it would indeed reprice the entire stay as though this was a new booking and if the rates of the other nights had gone up that would explain why the cost has gone up.

And as the poster above me states it's easy for the hotel to make sure you remain in the same room if you do have separate reservations (assuming they arer all the same type of course) either when you get there or you can email them a few days before youre stay starts.