What Causes Dark Spotting After Taking Duphaston?

hi, i took duphaston from my cycle 11 day till 25 usually my cycle 30 days after finishing the medicine on day 28 i had light pink brownish spotting the next day got dark brown kinda blackish the third day is the same and today its light brown not much just a bit,i usually have sore breast and painfull period but nothing this time around.what could it be?