When menstruation is low one month after taking birth control pills, there are many factors, such as pregnancy, because birth control pills are not 100% successful in preventing pregnancy, there is a 20% chance of failure, so there can be a chance of pregnancy. When there is pregnancy, the embryo can become stunted and stop developing due to the stimulation of the drug, and if there is a drop in progesterone, it can also cause a small amount of bleeding. If possible, buy a pregnancy test 12 days after intercourse to rule out the possibility of conception, and if necessary, go to the hospital for an ultrasound, because ectopic pregnancies can also cause a small amount of bleeding. If there is no pregnancy, consider the contraceptive drugs caused by irregular menstruation or side effects, so need to check the hormone six, if necessary, is to take oral cycle regulating drugs to repair the lining.