What Causes A Brown Discharge In Menstruation?

I usually have irregular periods but for the past 4 months, I have been having a long cycle ranging from 33-42 days. My luteal phase has been ranging from 9-13 days. At the moment I am on cycle day 37 (12dpo). I ovulated on day 25 and had sex on this day too. On 6dpo I had a temperature dip. On 7dpo i started getting a brown discharge and have been getting it for the past 4 days. Today is the 5th day and I am still getting the brown discharge(12dpo). The discharge is only a bit of brown spotting on my underwear and on the tissue when I wipe. I sometimes get brown discharge before my period but it normally only lasts for 1-2 days and then becomes heavy. What could this mean?