Well the last time I had sex, I started getting really bad stomach pains during it and they continued even after sex. This also happened to me in early may when I was having sex with a differant partner at the time...I had some pains during but it hurt really bad after aswell. Im also trying to figure out why this is...but it it's around the time you'd be ovulating (meaning about 14 days from the first day of your last period) then it could just be because were more sensitive there at that time.
Maybe you should go get tested for sexual diseases. I hope you used protection :/