Why don't you just ask her the reason she chooses to do this? I can tell you, being a human being myself, besides having lots of experience hanging around with.
I was thinking woah.. she's never licked my balls before. She must really feel dirty tonight. Then she starts kindof biting down on my dick real.
My suggestion is to take her over your knee and spank that bare bottom bright red anytime she does it, harder each time. If you go down on her, start biting and .