Girls, who have given birth or have already given birth several times …

Girls, who have given birth or have already given birth several times and who are on IUD, my period recently passed and for some reason it started to smear slightly with spotting that is bloody. I was scared, I went to take a pregnancy test, it's negative, what could this be? I will go for an ultrasound soon, because this doesn't stop!!! It has been going on for 5 days now.