Once in high school, I had a bandage on my arm from a cut that I made in the spring. I had a track meet so I had to go bare armed, and some people asked me what it was from. Me, not wanting to keep it to myself, said it my dog scratched me. After I said that someone asked, jokingly, "You haven't gone emo on us vj, cutting yourself, have you? haha" A few others jumped in too. I just said no, laughed it off, pretending it was all a big joke. I wasn't exactly your stereotypical cutter, so no ever suspected a thing.
Not to say people didn't know, they must have. I worked in a kitchen as a dishwasher, and was required to wear a t-shirt, so I would have 4 or 5 bright red marks on my arm that were clear for people to see at any given day during the during the winter of my senior year of high school. No one ever said a word, it seemed to be a very taboo subject.
People don't really understand it, as phocas said. Talking to someone about what is bothering you - if you can - may not be a bad idea. You don't have to tell them how you are coping with it, but maybe they can help you find ways to address what is causing you to do it. Feel free to talk to me if you want.