What Causes Heavy Vaginal Bleeding While On IUD?

Hi,
I have had an IUD for almost a year now and I never have periods anymore, just occasional brown discharge which I believe is quite normal. For the past few days I have been getting a lot of bright red blood 'discharge' and I have never had this before, it is not as heavy as my periods I used to have at all but I am not sure if this is normal and may just be my periods returning or if it is something else.
Thanks