take a test and call a nurse any nurse and maybe go to see a doctor.
there is no telling whats going on....you could be right and the uterus is just clearing everything out. some do get pregnant with it and keep the mirena in place til delivery because moving it could cause damages.
i never had clots til i started using birth control. they say you should see a doctor if you pass large clots, either way you need to be seen to ease your mind.
its really weird to feel the clots coming down, its also scary for people thats not use to it.