IUD Mirena pregnancy rates and feeling for strings

I have the mirena. I've had it since September of last year. I had sex with my boyfriend and he came in me for the first time. Who has gotten pregnant off of the mirena? Also, my strings used to be wrapped around my cervix and i used to be able to feel them but the first round we had sex I had this very strong abdominal cramping. So I went to check for them tonight and now I can't feel anything. The cramping has subsided now but every once in a while I'll get a cramp. Is it normal to not feel your strings?