She tried to resist it and ended up hurting herself
Considering most of us only know it's better to not resist solely from training in aikido, when showing anyone outside the art a technique that, if you feel resistance it's only because it's a natural reaction they haven't learned to control. Even if you were the grand master of kotogaeshi in all the world, she'd have zero idea how to receive it.
If you're lucky the Ladies of Aikido will comment on your dire predicament. As for her reaction, she was probably at first shocked to feel such odd control over her limbs/balance, and she most likely wanted that control back ASAP. The pain experienced by her was because you did not relinquish that control. You didn't mean any harm, but imagine that you didn't know aikido, and it was your girlfriend's older brother that wanted to show you this really cool thing called Kotegaishi. 1, you would look like a weak, awkward male, 2 it would be in front of the girlfriend, 3 it would hurt, and you would be experiencing someone else holding control over you that you did not want to continue.
Put this event in the context of the rest of your life for extra awareness body-building. what we're learning is enjoyable, but then so is rock climbing and downhill skiing. Respect it as potentially dangerous.
Good luck, and be happy if she talks to you again, and be happy every time she talks to you thereafter.