What Causes Irregular Periods After Taking Birth Control Pills?

hii i and my gf have sex on 23 jan and i have given him plan on same day but after that my gf got period on 31th jan then after on 15th feb then 1st march and then 17th march but her cycle was of 30 days before we have sex so can u tell me why she is getting period so fast and how long it will it to come in normal cycle..