I get the same pain in the lower part of my stomach, almost like period pain after sex. My doctor said its quite normal but couldn't give me an explanation so I am still wondering myself!
It could be because anything that actually stimulate your uterus can cause it to become irritable which can mimic menstrual cramps. It happens to some women.
I hear sex causes contractions which cause u to dilate :) my baby is at her lowest point that doctor could feel her head when she checked me. So she said sex sex sex is key to dilation.