What Is The Duration Of My Periods?

Hi doctor, I was confuse on my days of cycle. Can you help me to figure it out. I have my previous months period like august 11-17,2011, sept14-21, oct19-25,nov21-26,dec20-26 and this month January 28,2012 now is my 3rd day.. I'm confuse coz I don't know how many days of cycle I have.. Can u help me to calculate when will be my fertile days this coming month.. Thank you doctor