What Causes Brown Vaginal Discharge?

I had my period a month ago. Two weeks later I noticed a very thick (nearly solid), extremely dark brown substance in my underwear. I thought that I had started my period early, but it wasn't heavy enough to be my period. So I assumed that I was spotting. Since then I have continued to "spot" until three days ago when I started my period. I went on google to find out what could be wrong and some of the results were that I could be going through menopause, have a pregnancy complication, that I could have injured my vagina through intercourse, or be starting/stopping the use of a birth control pill. I am 14 years old, I am not pregnant, I am not sexually active, and I am not on birth control. Please help me figure out what could be wrong.