This is a difficult one to answer without seeing what is happening. The black dog thing could be a red herring except that black dogs are more difficult to read. The fact that you have added a puppy and he may be trying to protect her is probably also a red herring. More likely is that at 18 months of age he is now maturing and he is bolder.

What did the trainer say when it happened in class? Did they see what happened? do they have any 'real' experience in behaviour? I would be careful as many trainers say they understand canine behaviour, but it is very basic. personally, I think the best thing would be to look on the apdt website and find a local trainer who also does behaviour and get help. He's still very young and it has only happened a few times so can be dealt with. The last thing you want is him practising the behaviour and it becoming a habit.