Suggest Treatment For Thick Brown Discharge Prior To My Periods

hi i am 18 years old and i am on microgestin fe 1/20 and take it consistantly. but i had unprotected sex 3 weeks ago and about a week ago i had thick brown discharge for about 5 days (prior to my period) my period was supposed to start 4 days ago but when the day came it was just more discharge then it stopped. what does this mean?