Our dog growling at other smaller dogs

[Wildbird12]

We have an almost one year old Cocker Spaniel mixed with a Cavalier. She is a lovely, quiet dog. Excellent recall. We can happily leave her off lead at the beach or in the woods. She generally will sniff other dogs and move on after a few seconds. She'll sometimes play with other dogs. But will move on when called.

In the past week she has growled at two puppies on leads on different occasions. Both times she was on a lead and I pulled her away immediately. This morning she acted very aggressively towards a small dog - our dog was on lead. This is very unlike her. As soon as I said her name she stopped immediately and we moved on.

She seems absolutely fine with dogs bigger than herself. She plays with our neighbours dog regularly. We've put a lot of effort into socialising her.

She seems her usual happy self outside of these three incidents. Could it be to do with her just turning one year old?

If she's been on the lead all three times it could well be 'lead reactivity,' which is not unusual tbh - particularly in smaller breeds. When she plays with other dogs is she always off-lead?

I'd take her to the vets for a quick check - sudden changes in behaviour always warrant a vet check. If they can't find anything, it's time for a behaviourist to try and work with her to figure it out.

Personally, I wouldn't put it down to age given she's not at the right age (over a year, closer to 18 months or around the 6-9 month stage) where behavioural changes often occur - at least in my experience.

Just out of curiosity, OP, where was this? Is it a place you usually walk her?

I have a similar issue with my half-a-cocker spaniel - he's an absolute horror with puppies unless they're very robust or bigger than him. He's fine if we meet them on the street, but if we're in the dog park, in a play context, he barks at them - I think to play - and if they cower or run, he seems to think that's the game. He once chased a Belgian Malinois out of the park when they were both tiny. I now put him on a lead the second I see a puppy come near us. On the street, he can meet puppies and behave like a normal dog - in fact can be quite patient as they jump on his head. He is also fine with puppies who can stand their ground. Things I've considered: if it's predatory drift/territoriality/he's just a playground bully/it's a dog thing I can't completely understand but makes perfect sense if you're a dog.

I would absolutely love to understand the emotions going through his brain, but until I work it out, I just have to manage it by keeping him away from puppies unless I'm certain they can hold their own and/or he's not going to be a nob.