The vast majority of oral contraceptive pills do not bring on your period half a month early. If you take the emergency contraceptive pill, you will experience withdrawal bleeding about 3-5 days after you stop taking the pill, which is not a period. Retreating bleeding is less and clears up after 3-5 days, when it is time to have your period. Irregular vaginal bleeding may also occur if you are taking a short-acting contraceptive pill, often as a side effect of the pill. This vaginal bleeding is also not a period and is relatively small in volume. In most cases, the bleeding disappears after continuing to take the pill, so it is important to distinguish between vaginal bleeding caused by side effects of the pill and vaginal bleeding caused by withdrawal bleeding. If the bleeding is caused by either of these two conditions, it is recommended that it be treated appropriately or observed.