Hi I Have Been On The Pill Up Until May

Hi I have been on the pill up until May 2015 and since August 2015 I have niot had my period. I went to my GP and he put me on Metformin after doing a Glucose blood test. I was on metformin for about 2 weeks and I have had my period for the last 3 weeks, He now has out me on Cifran to control this. Not sure what could possibly be the problem now. Please advise