Our 7 month old Boxer mix pup had turned very aggressive over the last 2 weeks and nothing in our lives has changed. We have a very small chihuahua who is 3 yrs old. SHe has snapped at him when he walks by her bit our chi on the face causing him to bleed. She snuck a bag of treats I tried to take away from her and she growled very loud and bit my hand with enough force to leave a bruise. We had been able to take bones and toys from her before with no problem. Not sure why the mood change and what to do. I have grabbed her by her neck and held her on her back until she calms down but then 20 mintues later she goes after the chihuahua again. SHe stalks him like prey trying to bite his little legs. Any Sugg!!
Has she been spayed yet? Is she in heat right now? I know whenever our GReat Pyrenees came into heat she got a major attitude with the other animals in the house. Not to us, just the other animals.
Most likely it is due to her growing older and starting to test the waters and see what and how much she can get away with. Try googling NILIF and resource guarding and the ways of handling both issues.
Your chihuahua is not big enough to defend herself properly against your new dog. Thats obvious by the fact that your boxer has made him bleed once already. Unless you deal with that immediately there could be a very sad day coming in your future. If you haven't done so already, obedience classes would be a major help.