She's 3 and we've had her from 6 months, she's always been the softest non snappy dog I counted my blessings often... Yesterday out walking in a main street, on her lead of course, she just launched herself at another JR whose owner stopped to say hello, I'm sure she would have actually bitten if I hadn't pulled her back in time, just now, exactly the same at a large cairns(?) whose owner I know, just said hi to the owner and my JR flew at her, ears back, teeth bared, I actually thought she might have bitten her, but I'd pulled her back in time. Both times she remained very agitated, snarling, barking until the other dog left. I'm so upset, I never thought I'd see her like this, plus I have 2 young daughters who are used to her being as soft as anything. Have phoned the vets who gave me a dog behaviourists number, I've left a message with her, just anxious for advice, can dogs just "turn"?
If it's very sudden, it may well be that there is something underlying such as a physical problem which is causing her pain. Take her to the vets and ask for them to give her a good once over, including her teeth, ears etc.
Dogs don't just "turn" - there is always a good reason for behaviour, you just need to find out what it is. And even if she has a problem with other dogs, then there is no reason at all for this to include a problem with people.
First priority though, rule out anything physical at the vets.
I agree with the advice given by Scuttle - I've never known a dog 'turn' but am a bit familiar with this kind of behaviour in horses and the first thing we always check is whether the horse is in some kind of pain.
Also - I'm sure the behaviourist will dig deep and work it out - but was your dog trying to protect you for some reason? Does he think you're vulnerable at the moment? I fell in the garden yesterday and lay on the floor for about 10 mins trying to get my breath back and let the pain subside - throughout the whole thing one of my dogs sort of stood 'guard' straddling my head & I wondered if it was a protection thing. NB - my other dog had no idea what to do because he hadn't heard swearing like that before and a puppy I was looking after for the afternoon climbed on my tummy to go to sleep so who the hell knows what they're up to?!?
Monster, the point I'm trying to make is that you may not actually need a behaviourist. Only when your vet has thoroughly checked your dog, and ruled out any physical cause do you need to move to this stage. A dog with toothache or an ear abscess will be very grouchy indeed and may well demonstrate behaviour that is completely out of character. If the behaviourist doesn't ask if you've ruled out physical causes, then I'd be concerned.