It's not a "dominance thing". Dominance in dogs doesn't exist, and there's plenty of research that shows this. A good place to start: Dominance in dogs
Play-humping is like play-biting, it's just a stereotyped behaviour. Just as play biting doesn't mean your puppy wants to eat you, play humping doesn't mean your puppy wants to have sex with you. These behaviours are hardwired into all dogs, and they practice them during play. It just means she was "in the moment", having a really exciting playtime and happened to be in a position that triggered the "I should hump!" circuit in the brain.