IUD and having pregnancy symptoms like crazy.

I have had a Mirena IUD for almost a year with no side affects, no problems, and no real period in a while just a day or so long. I recently had unprotected sex and now I am experiencing pregnancy symptoms left and right. It has been 12 days since I would have conceived so I'm wondering what's up. I also can't find the stings to my IUD so I'm worried it's moved or fallen out without me noticing and I can actually be pregnant. I would be very excited just confused as to how it happened. 
Anyone out there that has gotten pregnant while using Mirena?