What Causes Periods Twice In A Month While Taking Metformin?

Good day, I was diagnosed with PCOS last April 2011. My OB prescribed me with metformin (500mg twice daily) along with Micropil plus (once daily). However, after the pills, I have been having periods twice a month. What does this mean? I'm in dire need of help since I am trying to get pregnant and my OB advised me to take this medication for 6 months.