Hi
Sounds like a Leash aggression to me. Dogs that shoe aggression or fear signs only on a leash. Usually caused by frustration when they are not allowed to move towards something they want.

There is an exercise you can practice at home that will make it easier for her outdoors. Put a target on the floor. A target can be anything she likes: a toy a treat... put her on a leash and start walking towards the target. If she keep the leash loose praise her and give her treats. If she pulls or bark turn around and walk away. Allow her to get to the target only when she is quite and not pulling.

I would try not to prevent her from things she want to get close to (like other dogs) but getting closer only when she is quite and reinforce for that.

I do recommend using a head collar