If you are taking the active pills correctly, then you need not worry. These pills are highly effective when it comes to preventing pregnancy. The effectiveness of the combination of progestogen-only and contraceptive pills, if used regularly, is around 90 – 91%. Which in other words means that only one in every ten women will become pregnant on this pill. With perfect use, these pills are 99.9% effective. The stress here is on “perfect use” which means taking it every day at the same time without any fail. If not, the effectiveness decreases and you might fall pregnant even on the pill.
That being said, there are other things which might make the pill ineffective. If you are having diarrhea or vomiting for more than two days, then the effectiveness of the pill decreases. The pill might not be very effective for very obese women. Certain supplements like St. John’s Wort and medicines such as anti-seizure medicines, griseofulvin, and rifampicin can also play a major role in reducing the effectiveness of the anti-pregnancy pills.
As @ness said, the best way to determine pregnancy is taking a pregnancy test. Ideally, this should be taken the first or second day when you have missed your period.
That being said, being pregnant is not the only reason why you might have missed your period. It can also happen due to extreme dietary changes, excessive stress, and excessive exercise.
Sorry for such a long reply, but wanted to touch all the bases.