I don't know. The more I'm reading the comments, the less I believe it happened when wiping. Perhaps that's when I first noticed the soreness, but the cut actually happened during sex.

Haha no it's not a regular occurrence, but it has happened before. My nails are never long, but my hands are dry. I don't know how it happened. Other than when I told the doctor it was the paper cutting me, he said it had to be a nail... my nails hardly extend the skin on my fingers.

Perhaps you have cut yourself, but never noticed, as it never caused you any pains.