Extreme cramping after sex

Has anyone ever had extreme cramping after sex? And not exaggerating on the word extreme. I had sex with my boyfriend last night and we were in the position he was doing me from behind, not in the butt but the vagina. It was a little painful during sex but he was enjoying the position a lot so I went a long with it until he finished, I wanted to. After sex he pulled out and immediately I had HORRIBLE cramps in my pelvic and vagina region. It felt like menstrual cramps but in a lower region and it felt like the worst cramps I have ever had. I took some tylenol and my boyfriend got me my heating pack, I was able to get comfy maybe 45 minutes after. I've had cramping after sex before but this felt like overkill, if I was my boyfriend I would've said I'm overreacting. Is this a sign of an STD or just a bad sex position?