Expert answer:
You can ovulate (but it is harder) even if you do not have menstruation or it is irregular. Probably this little dirt can not be considered a menstruation.
Metformin is an auxiliary treatment in PCOS (Polycystic Ovarian Syndrome), it does not mean that metformin will make your menstruation go down or even regulate it.
Also read the article: Is Metformin a Good Treatment for Who Has Polycystic Ovaries?