What Causes Absence Of Period After Taking Metformin?

Hi Dr. I am 29 yrs old and I do have a problem regarding PCO and was taking medicines for past 2 months and still continuing.The medicine is metformin 500mg and my LMP was on Jan 5th and I have not got my periods since then. I checked urine on the 50th day for pregnancy and it is -ve. Should I wait for some more days or what shall I do.