What Causes Brown Vaginal Discharge After Periods?

Hi, my age is 22, wt 115 kg and height 5'4. Ive been on the pill since 8 months. I hav a single partner n so does my partner.my period ended on saturday and then since the proceeding wednesday I.e. Three days after my period I started to have a brown vaginal discharge. The last time I had intercourse was on Sunday, the day next to wen my period ended. I am on pills and have been regular. I am just concerned about the discharge being brown. There isn't any pain or cramping or any abnormal amounts. It's just brown n sometimes reddish. Y could it be? I would also mention that I missed my non active pills during the period and instead started the active pills. Could it be cos of that?