Mirena uses the pregnancy hormone progesterone. I had the Mirena for 14 months and would have certain weeks on occasion when I'd get pregnancy symptoms such as morning sickness, fatigue, increased/decreased appetite, sore breasts, and cramps.
It's very unusual to get pregnant with the Mirena--not impossible, but not at all common. However, it would be wise to take a pregnancy test as soon as possible because if you are pregnant with the Mirena, it is extremely dangerous due to ectopic pregnancy risk.
You can get pregnant on Mirena, the only way to know is take a pregnancy test. But the hormones in the IUD can also mimic pregnancy symptoms for some. Take a test and if you are still worried see your dr. Good Luck!