What Causes Brownish Discharge From Rectum?

I am 31 years old,female. I always thought that brown spots on my underwear came from the early or late discharge of my menstruation. Little did I noticed, because of the pantyliner, that it is on the rectal part. I just confirmed it just these past two days that I stayed at home and didnt wear any pantyliner. What are the causes of these spots? are they blood or other substance?Is it bad?