What Causes Brown Vaginal Discharge?

For the past 2 days I have been having thick, very dark brown discharge. I ve never spotted between periods before (I m 20, got my first period around 13). There s been enough that I have to use a panty liner. There is no strong odor. Since this has started I ve felt very mild cramps or a mild dull pain that s concentrated in one spot but otherwise no pain, or itching/burning etc. I m sexually active but not on any birth control, but I always use condoms and it s never broken. I also don t feel nauseous or anything. My last period started 3 weeks ago and it s been pretty regular for the past few months. I have no idea what this could be.