Week after start metformin 750m.. for prediabet..started spotting 3 days. week later now period is late. met alter periods? no preg test yet.. no pms

Metformin: Metformin does have some beneficial effects on ovulation in patients with pre diabetes or Polycystic ovarian syndrome. Since you were only on it for 3 days the immediate effect on your period is likely coincidental. If you think you might be pregnant check then check a home pregnancy test. They are cheap, reliable and available at any pharmacy.