Hello, and I hope I can help you today.
In order to be effective, you must begin your pack of birth control pills, regardless whether or not you have sex, within 7 days of the beginning of your period, so you should have taken the first pill on the 18-25th. Birth control pills need to be taken daily to be effective, not just when you have sex.
So the fact that you started your pills only on the day that you had sex does not protect you effectively against
pregnancy, as you may have already ovulated this cycle before taking any pills.
At this point, the best thing you can do is continue to take your pill daily and if your next period does not arrive at the end of the pack, you should perform a urine pregnancy test if you do not menstruate.
If you do not conceive this cycle, and just begin a new pack of pills when this one finishes, you are protected against pregnancy any time you have sex. For your additional information, if you accidentally get pregnant when taking birth control pills, there is no potential harm to the pregnancy and birth control pills do not cause birth defects.
I hope this information was helpful and answered your question.
Best wishes,
Dr. Brown