My mirena IUD caused a lot of pregnancy symptoms vomiting, nausea, heartburn and mood swings. I had it for a little over a year and I was able to get pregnant with it so if it is different than what you've experienced before you should get a blood test.
I have no experience with the Mirena IUD (the one IUD I did have was the Paragard, and it was removed after almost 3 months) but it is well known at causing 'pregnancy' symptoms, even if it never has before. Heartburn can be from various things, and vomiting from heartburn and reflux is well known. Can you still feel the strings? If it's in place, the odds of pregnancy would be low. However if you do not, you need to make an appointment and have it looked at. While there you can also have blood testing done for pregnancy to make sure if you are or not.