female boxer agression/protectiveness after adopting a puppy

Question
We have a 3 year old female boxer, not spayed yet, and just adopted a 3 month old female boxer puppy. The older boxer is not aggressive to the puppy whatsoever, however she is aggressive towards any dog that she didn't know prior to our puppy arriving. I do not know if she is taking on the mother role of the puppy and being protective in her eyes or what. I can't take her around any dogs that she doesn't know though. I haven't tried taking the older female out by herself without the puppy though. What are your thoughts/suggestions? I would truly appreciate your input. I should add that the older boxer was never aggressive with dogs before.
Thank you very much for your help.
Christy smith

Answer
If she wasn't aggressive before the new puppy, and only against unknown dogs now, I would be inclined to believe she is protecting the puppy as an adopted mom.

I take it she has not had any litters as this is more common with females that haven't birthed.

Try taking her for a walk without the puppy and gauge her behavior.