Honestly, with the mirena it is probably more likely that you have some sort of infection or something. I had the mirena and am kid-free, like most women who have had it. (I hated it, but that's a different story) The mirena IUD is more effective preventing pregnancy than a man getting his tubes tied, if I remember correctly. This info is from the planned parenthood website.
"Effectiveness
The IUD is one of the most effective reversible methods of birth control. Of 100 women who use ParaGard or Mirena, one or fewer will become pregnant during the first year of typical* use.
Tell your clinician immediately if you have
* severe abdominal cramping, pain, or tenderness in the abdomen
* pain or bleeding during sex
* unexplained fever and/or chills
* flu-like symptoms - muscle aches, fatigue
* unusual vaginal discharge
* a missed, late, or unusually light period
* unexplained vaginal bleeding
Pregnancy — Most pregnancies happen to IUD users when their IUDs slip out without their knowing it. The chance that a pregnancy will happen while an IUD is in place is very low. If it does happen, have the IUD removed, if at all possible, as soon as you know you're pregnant. Women who choose to complete a pregnancy with an IUD in place must have close medical supervision throughout their pregnancy."
Once again, the chance you are pregnant is very low and your symptoms don't necessarily point straight to pregnancy. It is extremely unlikely you are pregnant, and slightly less unlikely that you have an infection. Any unprotected sex lately? Bladder infection? These could all cause a more internal infection that might be bad.
Good job seeing your doc. I hope it all comes out well.