The bleeding may be abnormal vaginal bleeding instead of normal menstruation, and a blood HCG test is needed to confirm the diagnosis. The pill is a highly effective progestin, which can inhibit ovulation to prevent pregnancy, but there is also the possibility of contraceptive failure. The amount of bleeding may be small and may be similar to the normal menstrual flow. A blood HCG test should be performed to determine whether the pregnancy is caused by contraceptive failure or withdrawal bleeding. After taking contraceptive pills, it is easy to cause irregular menstruation and it takes three months for menstruation to return to normal. If you still have irregular periods after three months, you will need menstrual regulation treatment. If you have withdrawal bleeding and then have sex, be sure to take contraceptive measures, any time you have sex there is a chance of pregnancy, because menstrual disorders ovulation will also be abnormal, emergency contraceptive pills can not be taken more than three times a year.