<human>: I recently starting hooking up with a girl I knew from high school. We are 22 now, and just realized we were a decent match.

It was our 5th (or so) night of sleeping together, and it was great. Semi-buzzed sex ending in mutual satisfaction. As morning rolled around, we decided to have another go. Things progressed normally, foreplay went well, and eventually we ended up with her on top. As she was on top, being incredibly sexy and making me realize I had landed something special, I decided to try and assert my male dominance. Mistake.

I thought a slap on the ass would be just perfect. Not too hard, but enough to get the message across. I'm a stud. As I swing my arm out to start building momentum, she sits straight up on me. My arm moves around and I start the slight whip-like motion so that it I get a nice "pop" as my hand lands.

I start to make contact. My palm lands directly in the middle of her butt cheek. My fingertips, fully extended, wrap around, only to find that I've overshot the runway (so to speak). My stomach drops, and my mouth starts salivating.

I slapped my balls. Hard. With the aforementioned pop.

I let out an audible groan, and she immediately knew what I had done. She tried to get off out of courtesy, but in my state, all I could mutter was "Don't move. Don't move."

I sat there for the next 5 minutes, inside her, trying not to vomit (successfully) and her trying not to laugh (unsuccessfully). 

After what felt like a lifetime, I was able to allow her to get off (not the way I had hoped).

Summarize the given text in a few sentences.
<bot>: I tried to slap my girlfriends ass while she was in cowgirl position, and instead slapped my balls so hard I almost threw up.