What am I doing wrong? Inconsistent Behavior on Leash

[winn21]

I have a five month GSD - very well behaved, adjusted and trained. She has been going to some type of obedience training since we got her and I also work with her at home. I have been "loose-leash" training her so our walks are mostly great - nothing unexpected.

At any of the parks, if she is off-leash, she is a dream. Plays, fetches, in and out of the lake and on the grounds - very attentive and well behaved. Social and nice to others.

But same park - whether near the water or not - on-leash her behavior is inconsistent and drastically different. She bounds, jumps, trys to get away, strains against the leash, thrashes around - I mean the first time I saw her like this I was shocked. On-leash (in a park) she is difficult with us, less sociable with others and sometimes aggresive with other dogs.

What am I doing wrong? She is such a good puppy - it breaks my heart to see her like that.