Some children actually cut themselves with knive or razor blades. They are feeling so much pain inside and can't talk about it. It is a way of feeling the pain and "letting it out" when they cannot contain it any more.
One of our children did this for awhile. Tell tale signs are sudden large amounts of "paper cuts" on the fingers. If you ask about it they may start doing it under their clothing. This is something that needs to be taken care of right away and I would immediately get professional help. Fortunately we were already in the midst of attachment therapy when it started happening so it was dealt with immediately. We started having him let me look him over while he was wearing a pair of shorts each night. Then every few nights. Then eventually it stopped. Educating him as to why he was doing it helped a lot and then in therapy talking about that pain.
Here are some articles on the topic. It is more common than you may think.
Cutting-Self injury from the teen perspective
Cutting: Warning signs and Treatment