What Causes Biweekly Menstruation Cycle While On Treatment For Migraine?

I am having my periods for every 15 days after taking some steroid for a month prescribed for migraine during June 2014, also I am using Copper-T. My Gyno has prescribed me sevista 60 one on monday & thursday but I took only 1 on monday but on wed. I got my periods. my last date was 29th Jan ( my condition is from July 14. Took ayurveda & was Ok for 3 months Aug 14, Nov 14, Dec 14