Hello and welcome to HCM.
Thank you for your query.
Mirena has a very common side effect of menstrual irregularities, this could be what you are experiencing.
It is also important to note that alcohol has shown to reduce the efficacy of mirena in a few undocumented studies, so kindly refrain from consuming alcohol especially when you plan on having
unprotected intercourse.
As mentioned, the symptoms you are experiencing could purely be due to the birth control, but
pregnancy will have to be ruled out either way.
Which is why I would recommend a home pregnancy test to be conducted either 2 weeks after your last sexual contact or on or after the first day of a missed period. The test would yield a most accurate result with an early morning urine sample. You also might have to repeat the test 2-3 times to confirm a result. Once pregnancy has been ruled out, I would recommend you to visit your doctor to confirm the cause to be mirena and not anything else.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications at: http://doctor.healthcaremagic.com/doctors/dr-shoaib-khan/64581 . I would gladly help you.
Best wishes.