I'm trying to understand something

Starting around the age between 7 and 9 years old, after I masturbate, I would slap myself because I thought what I was doing was wrong. I stopped doing that when I was 15 or 16. I now know that masturbating is natural, but I don't understand why I would think I was doing something bad. Either someone told me it was wrong and I shouldn't do that, or something terrible happened to me.

You certainly don't need to assume abuse or anything negative like that. When I began masturbating, I too felt terrible and guilty. I would certainly slap myself or do much, much worse to try and "condition" myself against doing it again. I never really stopped with that behavior of self-punishment. However, this is just an inborn aversion to what I was doing. Sometimes, we like some stuff and dislike other stuff. Simple as that. Brocolli is natural, doesn't mean I like it.
It's perfectly well within reason that you could have just naturally disliked this. Just because it's out of the ordinary does not necessarily mean that there was any adverse influence on your mode of thinking.
But if you personally believe strongly that there was a chance of abuse, then you should absolutely investigate further.