What could be wrong?

I know this is a little TMI but I need to know. I was masturbating, all outer action. Before I finish I begin to cramp at my lower stomach, upper vagina area and when I finish I have sharp cramps for about a 15 seconds in the same area. It feels like period cramps. I’m not spotting or anything yet, my period is scheduled to come on the 31st. What could be wrong?