Did my IUD move? Or?

My boyfriend finished in me 3 weeks ago, I took a pregnancy test that was negative (this was yesterday), and after about about 2 days (after he finished in me) I started getting cramping in my uterus which I know is too soon to start feeling things if I was pregnant. But, as time has gone on, the pinch/pain in my uterus hurts. I get constipation often, so, Ill lay on my stomach sometimes, and as I feel my gas move around, it hurts and pinches as well. Ill try to sit criss-cross on the floor and I cant sit long before the pain kicks in. I have an appt. for a doctor, but, Id still like opinions. Has this happened to anyone else? Should I see a general doctor? OBGYN? Could I be pregnant and everything going on is moving my iud?