Can you help me, please?

I have been dating my girlfriend for 8 months and she is the sweetest thing ever. She is 23 and loves kids and is really down to earth and classy. The thing is, she is pretty innocent and a "good girl." Whenever we are intimate or just acting flirty, sometimes I will call her my "bad girl" or my "dirty girl," and that will absolutely drive her wild. She gets the biggest smile on her face and she kisses me more passionately. Why would she do this when I call her that? Also, she doesn't ever cuss, but loves to cuss in bed. Why?