I`m wondering if it`s a bitch thing....
I have experience of 2 bitches who were apparently perfectly happy with each other for 3 years - then one day a squabble over a ball turned into a fight to the near death. Since then we haven`t trusted them together.

Definitely the vets and write down all that has happened so that you don't forget anything while you are there. Be as thorough as possible so that the vet knows all there is to know and runs the appropriate tests.

Good luck and let us know how you get on.

I would imagine at 6 years old and from a good breeder that it wouldn't be rage. This usually becomes apparent in the first 6 months, but can be upto 2 years of age from what I am aware. Not saying that it isn't something neurological, just saying that it proberly isn't rage.
I do agree it is probably pain related, and maybe the bopping around and being daft with you caused something to hurt more, and while your daughter may be very gentle and careful with your dogs, your dog may not view her as trustworthy as you are and so could have been tensing for that reason. Or it could be something totally different.
Vet first, let us know how you get on.