According to the data you provided, your girlfriend might not be experiencing withdrawal bleeding due to hormonal changes caused by the birth control pill.


Birth control pills are orally taken birth control products, which usually contain hormones. The hormones are used to prevent production of other hormones needed for ovulation, thus preventing release of an egg for conception. Many of them are taken for three consecutive weeks, and stopped for one week in which the girl is expected to have her withdrawal bleeding.


The withdrawal bleeding is needed to have the superficial inner lining of the uterus detached as in a normal menstrual period. Otherwise the inner lining tends to thicken, which, in the end, might turn into big bleeding.


However, the hormone changes are not always accurate, and the withdrawal bleeding might appear in any of the seven days expected. It might be even late for several days. In certain women, the menstrual bleeding might skip that time and is still not considered pathological.


If your girlfriend misses her period the next month, too, then she might want to visit her gynecologist in order to check if the birth control pills are causing significant hormonal changes and if another birth control pill would be more suitable for her.