She is 5. Fit and healthy. We were in a pub garden and another dog came up to say hello-they had a sniff and a wag- then she suddenly went for him. Other dog fought back but she definitely started it. Dp grabbed her and the other dog was jumping up trying to get her until his owner came out of the pub and got him. She has never, ever done this before- we meet other dogs all the time and if anything she is the submissive one. What shall we do? Could it be a one off- or should we muzzle her? 