What are reasons for hurting oneself and the seriousness of it?

Lately I've been kinda down and for some reason punshing myself to the point were i have huge bruises helps. Idk why I do this, but I want to stop. I've tried just going running and stuff but it didn't really make me feel better, and taking tylenol pm and sleeping it off, but it wasn't making me sleepy so that didn't work b/c i took a bit too much to the point were i couldn't sleep b/c it didn't really have the same effect.