Seeing your doctor is always a good idea when you don't feel like yourself.
Have you tried a home pregnancy test? You might get the answer you're looking for with one of these over-the-counter tests.
Chances of pregnancy are less in your case and the symptoms can be due to hormonal imbalance due to the Mirena IUD. However to confirm the status of pregnancy, I suggest you to get your serum HCG levels estimated since this is more specific than urine for pregnancy tests.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.