Unwanted bleeding after periods.

My dermatologist suggested me to take "Acutret"10 Mg & "Yasmin oral"at bedtime on 5 th day of period cycle. When i took acutret, i started suffering from skin infection & i stop using within 6 th day. And , On 6 th day of my menstrual cycle(Period) i took "YASMIN ORAL" pill (One pill) after a meal . Again , i stop using "Yasmin pill" within a day completely. As mentioned i only consumed single one pill of "Yasmin"(Birth control pill) on my last day of Period. Within a week i again started bleeding .... first two day was a spot bleeding and on 3 rd day i can see heavy bleeding as same like i get flow on my periods. Kindly suggest me ! why it happened?? as i only took a single pill of "Yasmin oral" birth control. I 'm tensed now. Note : My periods are regular and never ever such thing happened to me.