I have seen this type of behaviour from a collie v dobe before,collies tend to naturally herd in open field type situations and he was probably trying to do that and she was having none of it, the scream was probably down to him trying to pin her and the fact that she is still not mature as you say.Sooner or later she will realise that she is the big dog and then she will definately have none of it then you will have to careful she doesn't fight back if he tries to pin her.
