[Face Up.]

I think that this could be self-harm because you are inflicting pain upon yourself in order to "get yourself moving" which could be seen as a punishment. Even though it doesn't leave bruises, you are inflicting pain, and hitting is a form of self harm.

Perhaps you can find other motivators to get yourself moving, such as telling yourself you will reward yourself after you get something done. For instance, treat yourself to your favorite goodies or that movie you have been wanting to see as a reward, rather than slapping.