PCOS and TTC

Im a plus size women weighing in at 297 with PCOS iv lost 10ibs in the last month and working to lose even more. Iv been TTC for 9 months now with no luck. My doctor recently started me on Metformin to lower my insulin. But something strange happens two days after starting metformin. AF began i normally have longer but regular cycles that are 41 days long. There long but regular and alwaya show up and i ovulate every other month. Well this month it came on CD 28 and it was only 4 days long and i only had spotting that was light pink and brown that only lasted for a couole of hours then stopped. I called my doctor asking it metformin would start my period so early and she told me that it was not possible for metformin to trigger my period this early that it takes 7 days for metformin to stsrt working. Anyone have any thoughts as to why my period would begin so early!