What Causes Brownish Red Vaginal Discharge?

i am 20 years old my weight is around 90 to 95 pounds. My question is about my vagina. for about three weeks i have had this light brownish red tinted discharge. It has its moments where i though it had stopped and now its seems a little worse... it has a bad smell to it also . I have never had any thing like this happen before, i never really had a smell to my vagina. What is going on?