What Causes Delayed Periods And Brownish Vaginal Discharge While On Copper IUD?

Hello. I have a copper iud in for 3 years now. Always had regular periods always on time never had any complications. This month I missed my period. Now, a few days after my expected period I have very heavy dark brown and red discharge. Yesterday it was only a bit then suddenly today became heavier. I don t know what is going on. Took a pregnancy test told me it was negative. I have bizarre cramps and my breasts hurt the few days I was waiting on my period. Is this the result of a std or pid ? Anything helps thanks.