Welcome to e health forum.
Mirena is an intrauterine contraceptive that releases small amounts of the hormone levonorgestrel locally into your uterus.
It seems that you have severe anxiety due to no reason at all. This is probably due to the lack of proper counselling by your OBGYN about the effects of using Mirena.
Some common side effects of mirena include - missed or completely cessation of periods, irregularity in the bleeding pattern, pelvic or abdominal cramps, headaches, feeling low or depressed, Backache, bloating, pain during intercourse, vaginal discharge, breast tenderness or pain, etc.
Though most of these symptoms usually are more pronounced during the initial months after insertion, they can present even after several months or later. Onset of such symptoms causes confusion in many women to think about possible pregnancy. In such cases, a urine pregnancy test will help to clear the doubts.
For your information, chances of pregnancy while on levonorgestrel containing intrauterine devices (like mirena), is very less. The efficacy of mirena is over 99 %.
Pregnancy can occur only when the IUD has been displaced or has come out without your knowledge.
In case, you still have any doubts, you can consult your OBGYN doctor for proper advice and counselling.
I hope this helps.