What Causes Brown Color In Menstruation?

My period came november 12 and it was 3 weeks late.as soon as my period had finished, i was spotting a brown colour which wasn't normal for me. I went to the doctor and it turned out i was pregnant with an HGC of 25. i spotted the dark brown colour all through the pregnancy also. i ended up having a miscarriage at 3-4 weeks. which began on december 14. since then i have been bleeding, and this week i am spotting the dark brown color as before. i have had an ultrasound and the womb is apparently clear. i feel like i have been bleeding forever. is this brown spotting normal?