I don't know if this is similar to how you are as you say you are 'waiting for the bottom to drop out'....but I have hurt myself when feeling good also...I think, for me at least, it is about having any 'uncontrolled' emotion good or bad. I think being able to recognise how you feel and what that makes you think/respond is important...because self awareness can stop any impulsive actions and maybe if you get why you want to self harm you can think of alternatives...