This is my experience of metformin from a different angle but it may help explain what’s happening with you
I have PCOS, one of the symptoms I had was irregular, light but painful periods from I was 21 yrs old but was never on metformin for it.
Roll forward to age 46 and put on metformin when diagnosed T2. My periods became more regular and heavier but a bit less painful, my GP even suggested I may now need to take precautions if I didn’t want to be pregnant, I had “normal” regular periods after that right up until menopause