Main causes of spotting between periods could be due to pregnancy, birth control pills, infection in an area of the
body related to menstruation (including the
cervix and even the
bladder). Other causes could be polycystic ovarian syndrome, diseases of pituitary gland, hormonal imbalance of estrogen and progesterone and uterine abnormalities like fibroid and cervical dysplasia.
In your case, it can be due to the birth control pills. Stop the pills and see if it persists.