My 1 year old boxer Dori has suddenly become aggressive with other dogs. On Friday Dori attacked my 14 year old Bichon mix. While it could have been much worse (only a few nicks on the neck and face), I am very concerned that should they have another altercation Candy (my bichon mix), won't be so lucky. I immediately seperated the two dogs and they have not been in contact with one another since the incident. While Dori hasn't shown any more instances of aggression towards the other 2 dogs in the house, I am concerned that at any moment that could change. On Saturday, I brought Dori to a fundraiser for a local dog rescue. She was fairly well behaved although after an hour or so she started to act aggressively towards any dog that approached her. I removed her from the situation and she calmed down, no problem. Today my husband brought our 2 big dogs (Dori and our Irish Wolfhound cross, Sully) to a local dog park and Dori immediately picked 3 fights with 3 different dogs which is very out of character for her. Normally (as in up to last Friday), she is very friendly with dogs at the dog park and never picks any fights. In fact, she will often times avoid dogs that intimidate her and seek out ones she feels comfortable playing with. This is just so strange because this huge change happened in literally one day. Has anyone experienced something like this before with their dog? I've completed numerous obedience classes with Dori and know about all the triggers that spark dog on dog aggression and none of them seem to be involved with the recent turn of events. We are bringing Dori to the vet's today to make sure that her recent behavioral switch isn't due to a medical issue like pain or illness. We also have a consultation set up with a behavorist who is highly skilled in dog aggression (she is an expert advisor to the Canadian courts on dog aggression). This behavior is just so out of character for Dori and it is continuing to escalate. Any advice would be much appreciated.