Sudden aggression

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As you can see my user name, I'm trying to don a flameproof coat for this as I know the topic can get very heated. I've also cut it down but upfront wanted to say - yes I'm using positive reinforcement, yes I'm watching her like a hawk and yes now she's always on lead.

DDog is 6 a rescue been with us for almost 5 years. Always been a bit anxious, but in 5 years I can think of 2 occasions when she's snap - both reasonable at the time and the second was well over 3 years ago. She isn't overly friendly or social but off lead is happy to sniff about, do a bit of a bow, chase about and then has enough so potters off to sniff something or come hang out with me.

Until 2 months ago ... since then she's attacked half a dozen times, in fairness twice could have been resulting from another dogs behaviour but in the past for a similar incident she'd have just walked away - the others she initiated the attack. All the dogs were fine, I've checked with owners but can't dispute she was very very much in the wrong.

Been to the vet for preliminary exam, no health issues. Talked to the rescue who were helpful, spent a couple of hours with their behaviourist and she behaved beautifully so hard for them to judge but they had some good suggestions especially on reinforcing basic training etc. which we've been doing.

This morning, she attacked again unprovoked. I've got a behaviourist recommended by our vet coming next week (not in Uk so lock down doesn't apply).

I guess my question is more about me - I'm so heartbroken to think she can't be off lead, she loves mooching around the beach or parks and she's only young still. She's not hugely social but in the past enjoyed playing with other dogs, I can't imagine that not happening again. I'm trying to stay calm but I can feel myself tensing when we meet another dog which I know is the worst thing to do. I'm scared she'll genuinely hurt another dog, which would be awful for everyone. And I'm at a loss for what's triggered the change.

Just looking for advice, similar stories where things worked out and behaviour changed back. I know there are some behaviourist on this forum so any ideas from them even if just generic.

Is she coming into/out of heat or has anything changed in her routine/home life? X

This is a really good resource for identifying pain or discomfort in your dog

all-about-the-dog-academy.teachable.com/p/behaviour-problem-or-pain

Thanks for all the messages and suggestions on pain, muzzles etc.
Thanks @PollyRoulson for the Let's Go, the rescue recommended similar. Not working just yet it almost seems to make her more intrigued by the other dogs no matter how far in advance I switch direction.
Hopefully with the behaviourist next week I can come up with a solid plan to help her.

Start "lets go" at home in a calm environment with a fab treat, initially give her the treat even if she does not turn away with you .

If you introduce it only when you see dogs it is the warning to her that dogs are around so she will scan and look for the dogs

My terrier was super friendly until she was attacked by a pair of schnauzers, now she is very reactive. Almost like she wants to ‘ get in there first’
I walk her on the lead, crossing the road or keeping her close if we see other dogs now.
She’s only off lead where I am sure there are no other dogs, I take her to a nearby industrial estate on evenings and weekends and she has a good sniff around

Dog may be ill or in pain. Any sudden onset of a behaviour would warrant vet visit. Dog could have toothache, earache etc