What Causes Brownish Vaginal Discharge?

I have been having a brownish colored discharge for two days and about 2 hours ago it started to look like brown with a hint of rust color along with some mild cramps my last period was December 3 and before that I had been breast feeding as I had a baby on July 9 the breast feeding ended in November but I have never had this happen before even when I had a miscarriage I don't exactly know what this is