Help Me Define My Fantasy

I could use some help. When I masturbate myself, my mind wanders to fantasy like I'm sure most of us do from time to time. However, sometimes I can't help but fantasize about something rather dark-but it's only when I'm masturbating. I sometimes imagine that someone is cutting me open while I massage and probe my clit. (I know-it seems totally awful.) I imagine that my insides are being taken out of me, but I let it happen, I don't struggle, and I continue to masturbate until my final orgasm. Now realize, I DON'T WANT THIS TO REALLY HAPPEN TO ME! I am not suicidal and I'd like to think that I'm not crazy. Does anybody know if this kind of thing has a clinical name? Before I can muster up the courage to talk to someone about this I'd at least like to know something about it?

Anyone have any ideas?