I had my girl spayed at 2.7 lbs. It was REALLY scary but she was fine, and I'm so glad I did it both for her health and my state of mind.
That being said, humping is frequently a dominance behavior and she's just probably showing the cat who is boss. As long as the cat doesn't care it isn't really a big issue. Phoebe still humps her one toy, and occasionally Schroeder our other Chi, and he humps her. They were both spayed/neutered at the appropriate time - its just a play for who is top dog.
Honda certainly went to town though haha. Silly girl