thank you for your query.
Mirena is a intrauterine device
(IUD) with progestogen and is a hormonal intrauterine device.
what you are experiencing are classical effects of MIRENA seen in some people.
Mirena acts by inducing cyclical shedding of the endometrium.as you have endometritis
the uterine lining gets thickened and hence when mirena is placed,due to the hormonal action the lining is shed as clots.
And since you already got the pregnancy
test done it is the endometrial cast off.
All these symptoms are harmless if they are not accompanied by other symptoms such as dizziness
and pain in abdomen.
I would advise you to consult your OB/GYN if the symptoms persist for more than a week.and get an ultrasound
done to check for placement and condition of endometritis.
wishing you good health