Female Yorkie/Spayeding

Question
Dear Ann, we recently adopted a female Yorker from another family, she was almost 4 years old when we received her.  She was also on her last week of her cycle when we finally took her home and due to rules in our condo development we had her spayed.  When we brought her home she exhibited typical unspayed female dog behavior, she was all approaching to any dog, male of female but as typical she loved the male dogs.  All in all she was non-problematic and typical for the terrier breed, a little aggressive at times, she barked at strangers etc.  Issue:  We had her spayed about a week ago and ever since she has been horrible, she has become extremely aggressive, growls showing her teeth, and totally withdrawn as she mainly stays in her bed curled up, we initially thought she was in pain and/or had an minor infection from the operation, but we have since been giving her antibiotics and the vet gave us a 4 day supply of pain medication, we have been very conservative with giving her the pain medication, but we have given it to her.  She always was aggressive with possessions, and we understand this is not unusual for the breed, but she has now become aggressive in general more so with other people than with me, although now she has also started to growl and snap at me, to the point that I am reluctant to approach her.  One thing we have read and are doing is not to react to this and just come and go in the room where her bed is, she seems to calm down as if to wonder why we are not responding to her.  Prior to doing this we did respond to her by verbally speaking to her asking her "whats wrong princess?"  But I read that sometimes not responding can avert this type of behavior.  I guess my question is, what could possibly be wrong, since the behavior has intensified ever since we have had her spayed?

Answer
I think that her spaying may simply have coincided with the "honeymoon period" that is typical of a lot of re-homed dogs.  Some dogs, for the first few weeks in their new homes, don't bark, don't show aggression, and don't cause any issues.  Then, when they feel more secure, they go back to acting in ways that are normal for them.  So, my guess is that there is a reason this dog was relinquished by her first family, and I recommend that you not assume that this is due only to the spay, although pain can make some dogs rather aggressive.  I think you would be well advised to start clicker training this dog, and to do some behavior modification with regard to her possession aggression.  Good book on that subject: "Mine! A Practical Guide to Resource Guarding in Dogs" by Jean Donaldson.  If you need help with clicker training, and there is no trainer close to you, try the lessons at www.clickerlessons.com.  The more you can communicate with the dog, the better off you will be.