I sounds like it could be her hormones acting up. It does not sound like a dominance thing since she has never done it before. I have heard of alot of dogs, both male and female, start humping shortly after a spay/neuter. Most of the time the behavior went away after a few weeks or month. I was once told by a vet that having a female dog spayed throws them into like an early minopause (sp?). I don't know if it is true, just what I was told.
