Subsequent to using the miconazole nitrate vaginal insert(egg) i had a bad discharge, why?

Vaginal discharge: Miconazole inserts liquify in the vagina, so maybe it's the medication. If not, your apparent yeast infection might have been something else; or you had both yeast plus another infection. Any new, abnormal, or unexplained vaginal discharge should be professionally evaluated. STDs (gonorrhea, chlamydia, trich) are a concern if at risk, e.g. new or unsafe partner. See MD if you remain uncertain.