Sex and cramps?

Samantha
I'm 16w1d and the day after sex my uterus feels sore to the touch and I cramp. No blood. Just period like cramps. I'm thinking maybe it's because I tighten up my pelvis so much during sex that it just strains my muscles a little? Wondering if anybody else has experienced this! I'm not sure whether to call the doc or not since I'm not bleeding with the cramps!