E.T.
I did a quick web search and, yes, rarely women get pregnant while using a Mirena IUD. Call your doctor and find out what's going on. One woman had false negatives with home pregnancy tests, but another woman confirmed her pregnancy with one. I know that when I've been afraid that my doctor would think I was silly, he (or she, when I've had a woman -- right now my doctor is a man) has always been glad that I'd asked and has made me feel almost as if it would be silly for me not to have asked! I hope your doctor answers the same way. Good luck!