Has anyone gotten pregnant with the IUD, and how did you know?

My boyfriend and I had sex on February 3rd. He came inside of me twice, but I have the Mirena IUD. I've had the IUD since December 27th. I've had HORRIBLE cramping since we had sex. I know spontaneous cramping is common after the IUD but all of that had stopped a while ago. I had not had a cramp since mid January, until after my bf and i had sex.We have been having sex regularly for the past month (we took some time off after I got my IUD because I was bleeding) but he has never cum inside ne until now. I checked the strings of my IUD and they are in place. My boobs (especially my nipples) have been REALLY sore. Even the slightest touch makes me whince in pain. How long does it take for the IUD to be effective, and has anyone gotten pregnant with the IUD and how did you know?