FML. Similar situation which happened a month ago. Getting into it with her on top. She asked me to slap her ass, and so i obliged. Harder still, again i did. She asked for a third time, except this time i nailed one of my testicles with an overarching finger.

I immediately turned towards the affected side throwing her off my bed and onto the tiled floor. I hit myself so hard that i was worried i might have to check myself into emergency. It hurt for a month straight. She's fine too.