The only way to be certain is to see your Dr. for bloodwork. Any form of birthcontrol can fail, so it's certainly possible, although probably unlikely. Good luck.
I've had the MIrena IUD for almost 2 years (will be in January) and I've read most of the posts referring to "IUD Pregnancys". A couple of months ago my period went from normal to abnormal (as I would start around the 6th of every month, now it is anywhere from the 25th to the 10th) within the past week I've noticed a metallic taste in my mouth, sensitive breasts, increase in appetite, and extreme tiredness. I know these are signs of pregnancy but I am scared to take a test...advice?