I am sorry you have this THING in you body (all tho its your choice). The Mirena can give you symptoms of pregnancy, Mirena can stop your period or make it very light. I had it for 3 years. The worst 3 years of my life. I had it taken out on the 27th of August last year. You should google the effects that the manufacturers of Mirena don't tell you about.... good luck :)..
I doubt the pregnancy test is giving you a false negative. What you ate describing can just be normal for mirena since it has so many hormones. When I had it in I'd be regular and then randomly I'd miss my period for a couple months. Ended up getting it removed because I need a regular cycle for my sanity.