How long has it been since she passed the egg and for how long was she bound? It is quite likely she is merely seeking a quiet, safe place to recover - the nest box provides that. Any time you are concerned about a bird it can be helpful to set up a hospital unit in a place where they can be easily monitored -- ie in the house -- a small area that can be set up where they have calm, quiet, dimly lit space in which to rest and recover -- the isolation also makes it easier to monitor food/water intake and the output of waste, which are important to assess when concerned about their health.
