I had my Mirena in a year before I noticed the throat problem. It felt like the lump you are discussing, but it was also like I couldnt get a deep breath at all. Went to the throat specialist and the gastrointerologist...no one figured anything out. When I got the Mirena removed, I no longer had the problem breathing, but the acid kicked up and I could actually taste it in my throat.
I think it is different for everyone since our hormones are all trying to regulate. Hopefully this Mirena hell will be over one day.