My 1-year-old female Jack Russell has started a new strange behaviour over the last couple of days. When I sit on the sofa, she jumps up and "hugs" my arm with her front legs, just as if she were a male dog mounting me. She has this really intense look on her face when she does it. What does it mean? Why does she do that? I've never seen her do this before. (Just FYI, if it's relevant, she's spayed). I'm baffled.