My girlfriend wants me to slap and hit her while we have sex. I don't want to hit her. How do I deal with this? What might be the reason for her literally begging for such a weird thing?

We have been in a serious relationship for 2 years. We began to have sex about 4 months ago. In the past three weeks she began making these requests to slap,hit and pull her hair everytime we had sex. I love her more than I love myself and I can't even think about inflicting pain on her. Two nights ago she asked me to hit her. Wanting to make her happy I just gave a gentle tap on her buttocks. She egged me on to hit her with more and more force until she was red and squirming. She said it was what she wanted and it turned her on. I felt terrible about it and couldn't sleep. I cried in the shower today because I felt sick about it. I don't know how to face her? Should we seek counselling of some sort? Why is she acting this way? She was reading the 50 shades thing and asked me to read it too. I think she's wanting to try that kind of stuff. But all that I have ever wanted is to treat her like a princess. I am very confused. Please help me.

P.S: I have spoken to her. But she keeps saying that she loves it and I should make myself comfortable with that fact. But I really can't see her in pain let alone be the one who causes it.