"Humping" Lucy

I have a 6 pound dog named Grace and a 9 pound cat named Lucy.  Suddenly one day after being outside on a poddie run, we entered the house and Grace proceded to chase Lucy down and "hump" her vigorously, even breathing hard while at it.  I truly had never seen such a site.  My daughters friends laughed far too hard and had grandeur ideas of putting this cat dog "humping session" on YouTube.  Of course I renounced such an idea.  The "humping" has gone on for about two years now.  Lucy could run faster or jump higher but she choses not to.  Well on occasion she decides she is not in the mood.  Is there such a thing as a "gay" dog and cat deal?  Sshheessh. I conquered with our Veterinarian who reasoned this to be a heirarchy issue...Grace wanting to be dominant. Ya think?  Well a few days ago I observed a 180 on Lucy's part.  She suddenly wants no humping from Grace.  In fact she is swiping at her and hurting her. Menopause? And now Grace is having some asthma issues over all this relationship "stuff." Could be fear! I would say I seriously need a doggie/kitty psychiatrist for sure.  What am I to do?  The household dynamics has become quite dramatic.  Maybe Lucy needs to see the vet? She isn't herself.  : (  On the positive "end," My Lucy has her very own private Hello Kitty Vibrator!