Most pill packages have what we called "the green pill" during the last 7 days of your cylce usually when you have your period. They were like sugar pills and did not have any hormone in them. Alot of girls would skip those and wait a week and then start a new pack. That is okay. And, if your pack has just three weeks of pills and you started a new pack during your fourth week you should be okay. Your period was probably real short. You may have an irregular period this time . If you continued to take your pills it wont hurt you. But, just remember, lots of people have gotten pregnant while taking the pill. Its not 100% fool proof.
Hope this helps.