What Causes Prolonged Vaginal Spotting And Cramping After Starting Metformin?

I started Metformin 2 weeks ago on 23rd Feb, however a week before that I had my period from medication that forced it on due to me having polycystic ovaries. A few days after starting metformin I began spotting then it turned into a period but not very heavy. Since then I have been spotting and cramping, is there a reason for this ?