Small dog unreliable on walks

[OhCobblers]

So we have a tiny terrier who loves everything and everyone. Honestly anyone who meets her loves her.
We've trained her since day 1 and on walks generally she's great with other dogs. Either ignores them if we tell her to keep walking or plays with them.

If they're small dogs though recently she's had the habit of being aggressive if they get too close and sniff. She'll run at them being snarly and growling and I don't know why?

What further training could I do?
We're in the country side so want her to be able to go for walks off lead but it doesn't make for a relaxing time? She is 2.4 years (and has not yet been spayed if that makes a difference).
Would love any tips / advice.

Is there a reason you've not had her spayed?

I would be keeping her on a lead around other dogs for now. If she's off lead and bites then you could be in big trouble.

There was a couple near me who I passed on walks who had two cockapoo/labradoodle type dogs, one with each of them. Both dogs were very reactive to any passing dog and they would rev up as soon as they saw an oncoming dog and bark and lunge at the end of their leads. After seeing them several times and finding this very sad for dogs and owners (who seemed to be struggling - this was after/during 1st lockdown so they couldn't go to training classes), I saw them approaching a long way down a forest path. As soon as one of the owners saw my dog and me in the middle distance, I could see her tense and start shortening her dog's lead, then call out a warning to her partner. As we drew together, they stopped on one side of the wide path, braced to hang onto the dogs as they lunged. I stopped on my side, opposite them (I have a very calm dog) and suggested we just wait and ignore the barking of their dogs and see what happened (we carried on a calm conversation the while). My dog sat next to me gazing on them, doing nothing. After a couple of minutes both of their dogs stopped barking and lunging and sat and looked at us. They didn't bark again all the time we conversed. I pointed out that the dogs had stopped because they were getting no reaction from any of us. Unfortunately, the owners still looked worried and afterwards I thought I should have offered my dog and myself as walking companions next time we met (I also forgot to tell them to praise their dogs as soon as they had stopped barking - could kick myself). I never saw them again.

So, long saga but just try NOT walking past the dogs she is reacting to next time. See if you can get the other owner (as long as their dog is not retaliating) to kindly stop for a few minutes so yours will, hopefully, get fed up with barking with no reaction from anyone and being ignored. It is also quite important that you try not to tense up in anticipation of a "scene" with the oncoming dog as your dog will sense that through the lead and it will make her more excited. If she stops barking, lots of praise and a treat. Worth a try.

I know nothing about dogs... I walk other people's dogs who can be reactive and thought this was worth a try. It's not like I can do a lot of work with either dog.

I coo at them nice things as we approach other dogs, and praise them if they don't react. Mild scolding and continue on quick if they do react.

I may be deluding self, but I feel like both dogs have calmed down & are less likely to bark & get agitated than they were before.