New house, new puppy, old dog

Question
Hello,
I'm in a bit of a situation. My girlfriend and I recently moved in together.
I've brought my almost 3-year old female boxer who had spent essentially every
day with my sister's black lab as we were roommates for 3 years. We've only been
in the new house for about a week. I was afraid of seperation anxiety, but my
boxer seems to have acclimated herself well to the new surroundings. She's
clearly still adjusting but much faster than I had anticipated. The situation is
that my girlfriend brought home a new puppy (3/4 shih-tzu, 1/4 poodle). My boxer
wags her little tail furiously and seems happy, but also seems overly curious
and is quick to tower over the puppy. She will lick it and we keep her leashed
while she explores the new puppy. However when the puppy makes sudden movements,
my boxer will nip and bite at it causing the puppy to yelp. My dog has always
been the sweetest thing and it's difficult to see her cause another dog harm,
especially since I'm kind of stuck having them becoming housemates. I'm not sure
if she's playing, establishing dominance, is jealous, or is just plain making it
clear that she is not having this dog in her life. She's gone from having a 100
lb. playmate to a 5 lb. new addition in the span of a week (not my choice....my
gf surprised me with the addition as our place allows pets and she always wanted
a puppy of her own). Needless to say the potential of the dogs not getting along
is putting a strain on our relationship as she's sleeping with the puppy in one
room and I'm with my dog in another since we're afraid my dog will eat the puppy
while we sleep. Suggestions? Advice? Recommendations? Anything would be
helpful.....I hope this situation can be remedied.

Thanks in advance,
Anthony  

Answer
OK first bit of advice - get the dogs out of the bedrooms.
Its not good for dogs psychologically speaking to sleep in the same room as their owner - especially not on the bed.  Get a crate for the puppy and put both dogs in a small warm secure room with comfortable beds.  There will be a lot of noise at first as they protest about the new regime but stick with it, its much better for them in the end.  It will also be better for your relationship as you and your girlfriend will be able to sleep in the same room again.

As far as the two dogs getting along is concerned, your bigger dog is not being mean or vicious, she is just playing with the pup as she would have played with her larger playmate, unfortunately she does not know any different and big dogs will play very roughly with each other.  It is unfortunate your girlfriend didnt tell you she was going to get a puppy as a large breed puppy that would have ended up the same size as your boxer would have been much more appropriate.  However now you're stuck with it unless you can persuade your girlfriend to rehome the pup and get another of a more suitable type.
You will have to retrain your boxer to play gently with the puppy.  First off though dont keep her on the leash, this will make her more wary and more likely to nip and bite.  Sit with them calmly and encrouage calm and gentle contact between the two.  If the boxer gets too rough remove her from the room immediately and leave her in another room for a few minutes on her own to calm down.  when she is calm let her back in the room and allow her to play again.  keep toys away from them so that fights dont develop over those, and make sure all interaction between the two is calm and quiet.  
When the pup gets a bit older they will be able to play more normally but with such a size difference will always have to be watched in case the bigger dog gets carried away.  I dont think you have to worry about the boxer hurting the pup badly, right now she is just confused, the situation will come around eventually, but it may take a long time.
Hope this helps a little
Sarah