I think you should see your GP as this sounds more like a gynaecological issue than a diabetes/metformin issue to me. Starting metformin may have been the thing that tipped it over the edge, but it may not be the cause of the issue.
Cerazette is one option for treating heavy bleeding but I wonder if your doctor(s) have considered what is causing the heavy bleeding. I don't know how old you are - some conditions are more likely at a younger age, others in women over 35. It's possible that cerazette is no longer enough to control the heavy bleeding and other options might be needed. Did you have much pain with the heavy bleeding?