I'm 18 and I used to self harm. I kept it quiet for a good while but my mum noticed and went ballistic and slammed my arm in the door (I was handing her a towel into the bathroom when she saw) and almost broke it. She then made me go and tell my dad who promptly smacked me across the face.
Try not to get upset with your DD. It will only make things worse. I went through stages where my need to self harm would flare up and I would really hurt myself. I was being bullied at school, had been sexually assaulted by a "friend" and was having a terrible time at home (my mum had manic depression and my sister and I didn't know). I felt that there was nowhere I could turn to.
Make sure your DD knows that she will be able to talk to you about what's bothering her, because something is bothering her. If you keep that channel of communication open between yourself and your DD you will be able to nip any issues in the bud. My reason for self-harming was that I was feeling so much pain on the inside that I just couldn't cope and I got a release by cutting myself on the outside.
When my parents found out they did nothing to help me and assumed I was cutting because I wanted to be "cool". It couldn't have been farther from the truth. I was, and still am, suffering from clinical depression and needed help that I just wasn't given which just made it ten times worse. When my parents found out about my cutting and didn't do anything to help me it reaffirmed my (incorrect) belief that they didn't care about me which only made me better at hiding my cuts.
If you get worried that your DD is self-harming and claiming that she isn't, common places to cut are on the soles of feet, stomach, inside top of arms (covered by short sleeves), thighs and hips. I would suggest insisting on checking these areas if you become increasingly worried, but dealt with properly it shouldn't come to that.
Sorry if this has come across as a bit too heavy, but if someone had been able to explain to my mum what I was going through when I was going through it, my self harm might never have become as severe as it did.
Best of luck 