Myself and my 13 yr old son have this too... I find it 'calming' in a way and find I do it the worst when I'm stressed and/or uncomfortable in a situation (or just with life in general) LOL.. heck this whole evening has been terrible with it. but the bleeding part, although it sounds "gross", is what I look for. I need the pain from it to feel alive and the blood to show it... My T says it is a form or SI and I have found meds to be of no help. You're are not alone!