What Causes Brownish Vaginal Discharge?

I am having leakage from my vagina and its not my period because its not around my time. there's no way I'm pregnant so i have no clue what it is sometimes it looks like period blood but other times it looks more like muddy water. I'm not having any pains or anythings else. I am 16 and weight about 97 pounds. I have had no other problems