Thanks for the query,
If you have been 3 weeks delayed for your menses the pregnancy test is negative , then it is for certain that you are not pregnant. The most likely cause of delayed menses is probably stress induced hormonal imbalance which can cause delay in the menses. It is suggested that you should undertake an ultrasound pelvis to look for endometrial thickness which if found to be more than 10mm, then you can opt for a withdrawal bleed with progestins to resume your menses naturally.
I hope i have answered your query in detail,
Wishing you good health,
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