Could implantation happen 12 days after iui? i had some brownish mucus when i wiped. what does this mean? i also took a test last night and it was neg

Not likely: Implantation typically happens about 6 days after ovulation occurs so 12 days is a stretch. How certain are you that the IUI occurred at the correct time? Were sonograms being done. Was an HCG trigger shot used? The brownish mucus more likely represents the beginning of your menses (so the IUI didn't work). The negative pregnancy test seems to confirm that.