I am placed on metformin this month though not a diebetic patient but trying to concive (trying to be pregnant) and when it is time for my mensis I saw dark brown or black color inside my vagina , this is the third day, it is not staining or droping out on my underwear, it is just inside, i see it only when I put my finger in. I have done pregnancy test and it read nagative so i just want to know if it is normal Thanks