Suddenly aggitated behavior???

Question
While I know their is no "quick-fix" for my dog, I am becoming increasingly concerned-- No one is in danger, and certainly not the dog, but she has slowly become more and more "sensitive" to her surroundings.

She is a chihuahua, and very well mannered. She is a year and a half old, and we live in an apartment situation, at the top of a three story complex-- and have lived here about a year. Before this place we lived at another apartment complex in the same setting. Since living here she has suddenly become frightened of "noises" that we do not hear (I assume they are noises...). Occasionally I will peep out my door to see if anyone is there, and have even walked down the stairs. When I approach the door to inspect the "noise" she gets more aggitated, barks louder, and growls.

So now, I have stopped approaching the doors or windows to inspect-- thinking that maybe it is re-enforcing her behavior. And instead call her to sit next to me on the chouch/or bed--where ever I happen to be. But still, she barks. Even when repremanded with a short squirt from a squirt bottle, or verbal "shh!" Normally any of those things would get her to stop, but when she gose on "patrol" I might as well forget it.

I spend much of my time at home, she gets plenty of play time, social interaction--- and I just don't understand where this behavior suddenly comes from. Her favorite place to bark is at the wall behind my bed-- on the other side of that wall is the stair well outside.

This all began since being spayed (about 2 months ago now), and displays more aggressive growling when playing... is all this new behavior a "challenge" for dominance, or is she simply paranoid??

Answer
The incidence of unwanted behavior in a bitch spayed before her first heat cycle is large enough to advise that every bitch be allowed one heat cycle (providing of course owners are vigilant against her getting pregnant!)  The increase of threat of mammary cancer is minimal.  Your dog may have been spayed at a time when she was approaching her first heat cycle and is stuck with estrogen charged behavior.  This may account for her "aggressive" growling during play, or it may simply be that she is testing you because you are using negative reinforcers (like the spray bottle) which disrupt trust.  I can't put hands on this dog but I very much doubt there's any dominance involved here.  The Chi is a candidate for fear aggression for a great many reasons, not dominance aggression (excluding rank opportunism, which is the fault of the owner).  Regarding her play behavior, develop a "take it/leave it" game: play with her for a few seconds, then stop and simply hold onto the object with no reaction.  When she drops it (just as she drops it), say "Leave it", praise her, then offer it back with a "take it"; repeat this a couple of times a day for a few days and she will begin to relinquish the play object when you say "leave it".  Even though she may still growl 'aggressively' during play, you will know (and be less anxious because you know) that this is merely play behavior and nothing more.  If you've ever watched two puppies tug of war with a toy and heard the escalating brouhaha of growls and snarls, you'll know what I mean.  Play is social interaction but also a test of strength (and rank).  A dog that will relinquish an object willingly on command has proven its intention is benign.

Regarding the barking (especially directed at that wall), there's definitely something going on and my guess is: unwanted little "visitors" that you do not hear and cannot smell.  Commonly a dog will respond to a "quiet" command after the human inspects: dogs raise alarm for the top ranking member who then will go to inspect.  In this case, her prey drive may be engaged (making it difficult to disengage her behavior) or she may simply be afraid (also making it difficult to reach her.)  Try using some play training and a special toy that's created as a reward.  See this link: http://www.dogplay.com/Activities/obedience.html

Redirecting her attention to something that excites her response and is also rewarding is far better than "punishing" her (by squirting her) or offering a "quiet" or "shhh" which can't punch through the prey or fear response.  The play training object does engage the prey response and is also a palliative for anxiety, with the added plus of giving the dog an intellectual exercise (obedience response.)  The dog is not paranoid.  Something IS there.  You might ask neighbors in this complex if they've had problems with mice; these little critters can be so canny that you would never know they are present and stairwells, perimiter hallways, baseboards, etc. are all wonderful roadways for them.  If you can't break her of this behavior, go to the veterinarian for neurological evaluation and ask for a short course of medication that can help her be calmer and respond better to your play training redirection.