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!
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.