I Have Had My Mirena IUD In Since My Son

I have had my mirena IUD in since my son was about 5 weeks old, he will be 4 12/12/15. I have been experiencing dark brown/blackish red spotting on occasion, spotting every few months. Is this normal? I'm getting it removed tomorrow because my fiancé and I are going to try and have another baby, but I just started spotting yesterday, and for 2 weeks before that was experiencing very sensitive breasts/nipples. Could these spotting occupancies be my "periods"? And how long does it normally take to be able to get pregnant after IUD removal?