What Causes Brown Vaginal Discharge While UPT Is Negative?

I am 21yrs old, 5'2 and about 155. I had my period about 2 1/2 weeks ago. now i have had chocolaty brown, chunky discharge for about 1 1/2weeks now. This is not normal for me. i just tested negative for chlymidia and gonneria and pregnancy about 4 1/2 weeks ago. i am sexually active. please help me narrow down what could be wrong