I got pregnant with an IUD

After my 3rd child was born in 2015 I opted to get the mirena IUD. My doctor raved about it and how great and effective it was! It was perfect for us we knew we didn't want more babies. A few weeks ago I went to my doctor for my annual check up. The doctor knocked almost as soon as I finished up giving my urine sample. She said do you know your pregnant?!? Umm no! I had to immediately schedule an ultrasound to check that it wasn't an ectopic pregnancy and see where the IUD was. Fast forward to the ultrasound. The tech gets the wand or whatever it's called for the transvaginal ultrasound she immediately takes it out saying "oh I can't use this you're too far along". Excuse me?!?!? She did an abdominal ultrasound and said to me it looks like your about 18 weeks. At this point my head is spinning!! When I stopped getting my period I thought it was from the IUD because my doctor had told me to expect it to stop coming. I'm due June 22nd 2017 with our 4th child and to say we're shocked would be an understatement. I just can't believe it still! Anyone else have anything similar?? Please tell me I'm not alone!