What Causes Abnormal Vaginal Bleeding While On Implanon?

I have had my implanon in for nearly 3 years now I have about one month left and about a week ago I just finished my period and had unprotected sex 2 days after I finished. Its now been 6 days since I had sex and now have started bleeding again but its more a brownish red color. Not spotting or light. Could I be pregnant or could I just be getting my period again.