It could be caused be many possible illnesses. It could also be internal hemorrhoids. Internal hemorrhoids come with other symptoms as well, including bleeding while passing stools, residual blood that could be seen in the toilet bowl or wipes, burning sensation, itching, rectal discomfort and pain that is usually associated with the cleaning of the anus after moving bowels.
It's better to have a stool exam and be checked by a doctor to confirm this. And to be safe, avoid food that could trigger hemorrhoids.