Vaginal Cut after sex?

My SO and I have never had this problem before, so I was quite surprised when the other night when there was a significant amount of blood after sex. I thought it was my period (it was, though it's light since I'm on the pill), but there was a stinging sensation. The next morning I noticed a paper cut sized cut on the inside of my outer labia. Has this happened to anyone? If so, is it something to be concerned about, and how should I treat it?