What Causes Brownish Discharge During Menstruation?

My period started on Aug 17th and finished about a week after. for the past 2 days i have been producing a brown discharge. I am sexually active but use condoms. however on the last day of my period my boyfriend entered me unprotected but only once and then we put a condom on and actually had sex. everything on the internet says im pregnant. please can you explain what is going on