[Jess~]

self harm is all about intent, not the actual act of being harmed. if you're cutting vegetables and accidentally cut your hand, it's not considered self harm. even though you did harm yourself, it was an accident, therefore you didn't mean to do it.

so with that being said, what is your purpose for doing sit ups? is it to get in shape because you feel fat, or is it to make yourself throw up? if you know it's going to make you throw up and keep doing it to that point on purpose, for the purpose of making yourself throw up, I would consider it to be self harm.
likewise, if you are absolutely pushing your limit with doing these so much so that you wish you could quit, but you force yourself to keep going with the intent of punishing yourself for being "fat", that would also be self harm. I think that if you intend or want to hurt or punish yourself, it is self harm.