I had a similar feeling where my pulse would go crazy during position changes. My cardiologist thinks it's POTS. He said "trauma" to the body can cause this and the Mirena can be considered this trauma. It should go away on it's own. I also had orthostatic hypertension. It does feel like everything is getting better. I think it was a beta blocker they use for this. I just didn't want to go on anything because for the most part my BP is really low except for these strange times.
Also, something I think is interesting is that my Vit D was very low. There are new studies out that talk about Vit D and the role it plays in inflammation (including the cardiac system). They were saying that black people (who I guess don't absorb Vit D as well as white people) have a higher instance of heart problems. I wonder is low Vit. D (possibly caused by Mirena) can contribute to inflammation that affects the cardiac system and even other systems in the body.