What Causes Brown Spotting After Menstruation?

Hi! On October 29 i got my period, and it ended November 3. Yesterday I got some brown spotting after being clear for 6 days. I thought i was getting my period for the second time because it looked as if it was the "first days" of my menstrual cycle. Later I checked again and it was clear, no more brown blood. You should know that I am 18 years old and sexually active. I've only had sex with one person and he tested negative for any STDs. wondering if im pregnant or if anything is wrong with me?