hi,thanks for query.Normally after taking this drug the withdrawal bleeding should come in about 4-5 days,it is almost 3 weeks as you and pregnancy
test are negative.There are chances of some hormonal
imbalance in your case,it is better that you visit a gynecologist and get your self examined for this.A ultrasound
will also help in clarifying the doubts.At times too much of anxiety
,some other drugs taken etc ,can delay your periods apart from hormonal imbalance.
wishing you good health.