Mother nature

So i have a doctors appointment.coming up but i would like some feedback now.. Alright, to start i have the IUD - ive had it for almost 4 yrs now next year i have to change it. My ob told me it was supposed to stop my period but it never did. Ive maybe missed my period 2 ir 3 months out of these entire 4 yrs. Well this month my period came. It started out pretty normal - small amount of pink and red a little brown. Well as the days passed by period blood was so brown that my tiny clots were black. And im a little worried. Could it be some serious old blood that came out this month or is it something i should of looked into a lot sooner than my upcoming doctors visit!?