i have an iud for about 6 years now, my period used to be regular but lately I been having some changes, one month comes heavy and it lasts for 7 days and other month it comes light and lasts for 3 to 4 days, this month I missed my period for 10 days and I took 4 prednancy tests and were negative , I took pregnancy test even if I have the iud because 6 years ago I got pregnant while I had the iud in and I have a beautiful 5 years old daughter now . after 10 days from missing my period I noticed yesterday some brown to black spotiing , it is light but I have been having this kind of discharge since yesterday.I wonder what is going on, your advice please