IUD's are of many types it is of copper containing IUD's, hormone containing ones.
Since you are not pregnant as per your home pregnancy
test. It may be due to inherent hormonal
imbalance or naturally occuring hormonal imbalances.
Even though you have IUD in place, your regular ovulation cycle is not that much disturbed meaning you will be ovulating for which cyclic hormonal secretion will be occuring
These hormonal changes may cause discharges which may appear to be caused without no reason.
If the problem persists and cause serious inconvinence you may consult your gync. She may prescribe drugs to regularize bleeding