HELP ME!!

So I have the Mirena IUD and I have had for over a year and I have had a period. But this past month I was having brown discharge/spotting before my “period” which wasn’t like a normal period mostly spotting!! But I am still having the brown discharge/spotting when I wipe and it is always on my underwear. I took a pregnancy test about a week ago but it was negative.. i have been super horny lately like way more than normal, but I was like that when I was pregnant with my son!!which his he is a year and half now!!!! I don’t know what’s going on? What should I do!!