Implantation bleeding or Mirena

Isabelle
I am very new to the Mirena, only had it about 3 months now. The first period that I had was about 2-3 weeks long, and was very heavy at first and then was just some heavy spotting for a while. 
I was supposed to get my period a week ago, but I think that it is off in the app, but I am not sure. I have taken two pregnancy tests and both have been negative
This last week I have had some very tender breasts, and last night I had some pretty bad cramping. When I went to the bathroom there was blood, bright red looking, but then also some brownish. I put a tampon in (because of course this had to happen when I left home for a trip and only had tampons), and about an hour later I checked it and there was some bright red fresh blood and also some like brownish blood as well, and there wasn't very much. This morning when I woke up there was only some brownish blood. But the bleeding has seemed to stop. 
Has anyone experienced this with their Mirena? Does this sound like Implantation bleeding?