What Is Dark Brown Discharge Coming From Vagina?

Hello- I am slightly concerned about dark brown discharge I am currently experiencing during what would normally be my menstrual cycle. I have noticed very light brown spotting right before my period for quite some time now... but, last Sat night/ Sun morn, after sexual intercourse with a long-term, monogamous partner, my normal period have come down at regular flow (actually, during the intercourse). The next day, as I prepared for my regular cycle with use of a tampon, nothing came down but medium brown discharge. This continued over the course of this week, with heavy brown discharge and what appears to be lining shed (small shreds of particulars mixed in the brown discharge). What are some realistic possibilities here?