What Could Be Reason For Irregular Periods Even After Taking Primoult N?

I am 35 years old unmarried girl, form june 2012 my periods are bit disturbed, usually in mid of a month it starts bleeding, I consult gyne , she prescribed me Primolut n 5 mg I continued the course for 3 months and my periods were normal after few months once again in April 2013 I got bleeding on 18 day, in may on 20 day, I once again consulted gyne she said it is just due stress and once again she prescribed primolut I continued it for 2 months then I stopped it once again problem reoccur so I visited her once again she prescribed me for another three months. this is my third month of primolut , my periods was on 18 dec and I started primolut on 6 day twice daily, but on 11 day I got slight bleeding around 4 drops in a whole day still I continued my primolut today is 13 day still its bleeding slightly, in june 2012 ultrasound was also done and it was totally normal, plz help me what to do?