strangest behaviour I've seen in a girl...help

[luvofallhorses]

good thing that she's spayed. I have never had my girl dogs or other girl dogs do that to me..it's always the boy dogs..
..although it could be a dominance issue. when she does that to you just say "off" in a firm tone of voice and see if that helps. when she gets off give her a reward like a treat or just praise her for getting off.

[bckrazy]

Yes, humping definitely denotes dominance. When you're down at her level playing with her, she might be seeing that as an opportunity to dominate you with the humping.

To curb this behavior, I would definitely start NILIF, and give her a stern "No" when she humps and end the game.