it's the endorphins, that's definitely a reason I've heard before, Dagny.
there are so many ways I've heard people justify cutting, but in the end, no matter WHY you do it, it IS a problem, and you NEED to seek help for it. Psychologists are there for a reason. please take advantage of your resources (school counselors, even) and get help to turn things around. a supporting person that you can just talk to can mean the world.