What Causes Brownish Discharge During Menstruation?

I'm 37 & had twins two yrs ago . I started my period on July 10 -15 then on July 26 started brown discharge and have had it for 13 days and today it's turned to blood. I took pregnancy test and it was negative -I had polyps removed 3 1/2 yrs ago and was wondering if maybe those returned but I've never had this problem before .