pregnant????

Okay,  so I'm on the mirena IUD, & usually I get a period every 3 months or so, I haven't  had any problems with it, I love it ! But now all of a sudden I have been real nauseas, tired, & been having lower backaches.. my boyfriend and I have unprotected sex almost every day.. & im just real nervous..I'm going to take a test when I get the money, but my question is, has anyone ever gotten pregnant on the mirena or any other IUD? If so, how rare Is it to get pregnant on it?? Could it be possible I could be ?? I'm just trying to be prepared for when I do take a test & it says I'm pregnant.. I have a 3 year old son with another man, but I'm scared I could be .. HELP???!!!!!