We brought our dog home from the shelter around 3 years old, she had been a stray for about 2.5 years previously. She's 6 now.
We like to say she has a big personal bubble. Sometimes she'll sniff other dogs, and let them sniff her, but all of a sudden, she's done and tries to chase them off. She also can't really handle more than one dog at at time. She's generally more comfortable with dogs smaller than her (she weighs 35 lbs) but certainly gets uncomfortable around them too. Leashed or unleashed (i.e. a dog park) doesn't seem to matter.
When the other dog takes the cue, things are fine, but when they keep nosing around or getting in her face she gets really riled up. She has a few dog friends she acclimatized to and plays with fine, but we moved away from them. We got her used to them by leaving her with the other dogs for a week. Obviously, this isn't a good solution for dogs at the park or friend's dogs.
We try to introduce her to other dogs but we really can't predict when she'll be fine and when she'll not tolerate them well.
I'm not really sure what steps to take next to help her relax around other dogs. I've thought about putting on a comfortable muzzle so that she can see that other dogs aren't going to be aggressive toward her if she isn't but I also don't want to make her even more nervous.