What Causes Vaginal Spotting And Greyish Brown Discharge During Pregnancy?

I m about 4 or 5 weeks pregnant. On the 17th I went to the ER because I was spotting blood and had cramps. They said it was a threatened miscarriage, everything turned out ok. They also gave me a Rhophylac shot at the ER due to my blood type. About 4 hours ago I went to use the bathroom and noticed a greyish brownish discharge come out, but felt no pain whatsoever, I just want an answer on what this could be. Thank you