Hi I Am 18 Years Old And Had Sexual Intercourse

Hi I am 18 years old and had sexual intercourse Dec. 27 with condom use. A week later I got my regular period with cramps and as usual for a week. However two weeks after Jan.23, I a little bit of clear mucus discharge followed by dark brown vaginal discharge which lasted for a week. I took a pregnancy test and came out negative. But now its Feb 13 and I got the brown discharge again and I've had it for a week too, and I'm supposed to have had my period. This has never happened to me before and I'm worried please help me.