Yeah, you gotta be careful with body conditioning exercises like this. Your sempai shouldn't be slapping you so hard that it hurts like that. The slaps should
sting and be rather uncomfortable, but not actually seriously painful. Once it starts really hurting, you need to let your Sempai or Sensei know immediately, and they
have to either:
1: decrease the strength of the strikes and/or...
2: stop the activity for you and/or...
3: see how you are and if possible administer first aid
Martial arts teachers have the same legal duty of care just like school teachers. And quite frankly, being beaten up well beyond your pain threshold isn't that beneficial for you anyway -- cos otherwise if it were, you might as well ask Sempai to smash you a couple of times with a sledgehammer to toughen you up.
Like other forms of conditioning, it's gradual, like say doing weights. You don't start off at the extreme end and keep going - you start off small, and as you improve you
gradually step up. So in the beginning the slaps should be relatively light, then as your tolerance for the slaps builds your partner can slap you a little bit harder then increase the strength of the slaps as you become more resistant. And this may take weeks or even months to build this kind of tolerance.
Caught out!

Maybe you can convince her to let some students sew Transformer logos onto your gis.
