I have a 4 year old spayed yellow lab. She has always been sociable around other dogs and loves to play. However, in the last few months she has started to become quite grumpy/aggressive with other dogs when out on a walk e.g. if a dog barks at her, she will retaliate and bark back. The other week we took her to visit friends and they have a older retriever who she has always loved and played with. This time, play looked nasty and her teeth were on show so it was stopped. Last night, we stopped off at our local, a pub she has been in many a times over her years and another dog came in. Both wanted to meet each other so she was taken over and they sniffed, all was well and then suddenly out of no where our lab started barking and showing her teeth. I'm not sure why this behaviour has started now and wondered if it was just a case of her being territorial or if anyone had any suggestions on how to stop this etc?